The History of Biological Theories

'Practically, when a naturalist can unite ty means of connecting links any two forms, he treats the one as a variety of the other; ranking the most common, but sometimes the one first described, as the species, and the other as the variety* (Origin of Species, Since the practical man often finds it difficult to dis- tinguish between a species and a variety, the two cannot be fundamentally different. A variety is a minor species. There are many different categories classed as varieties; some are quite definite, others distinguished only by almost invisible differences. As we can point to no con- stant standard of difference, there can be no definite boundary between those characters that constitute a

variety and those which merely represent individual variations. 'From these remarks it will be seen that I look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, for convenience sake* (Origin of Species, 1895 edition,

C Darwin thus revived Leibnitz's formula Natura nonfacit saltus. This famous axiom was given currency by Linnaeus. It was common to all biological theories of the eighteenth and early nineteenth centuries, and it was given many different interpretations. Once more, Darwin made no attempt to discuss these earlier views, no effort to show how much of them would be superseded if his own theory of transitions were accepted. He rather treated them as mere anticipations of his own correct doctrine.

Ehrase Natura non facit saltus was used by him and by eibnitz to express widely different views. To Leibnitz, as to all eighteenth century thinkers, the phrase expressed the belief that the entire range of living organisms was an embodiment of the series of very fine shades of difference, in the Original Thought of which they are the varied manifestations, that is, they differ in essence. It was just this idea that Darwin and his followers combated in using the same words. They asserted that between isolated phenomena there are only unessential differences ; in Nature things merge by insensible gradations one into the other, just as, for instance, the hamlet becomes the village, the village the little town, the large town, the city. The distinction between these terms is merely a matter of arithmetic, or of an arbitrary scale in the minds of those who use them, s

We must particularly emphasize that to Darwin such quantitative differences were the only differences that 'existed, not only between species and varieties and indi- viduals, but between all things whatsoever between men and monkeys, between morality and immorality, between the tertiary formations and the diluvial strata and so universally. Darwin welcomed every new case which could be re- garded as an illustration of this teaching. He was content merely to prove that there are such transitions. He-knew that Linnaeus believed in the constancy of transitional forms, and had come to the conclusion that species are immutable, but he does not seem to have considered it necessary to inquire how he came to hold so very different a view.

2. Darwin went on to explain how links between species may arise. He shows, referring to many examples, that descendants of the same parents differ greatly from each other, as well as from their parents. Once more, he stresses the number of his examples without making any sustained endeavour to analyse them. Nevertheless, he gives the following causes which will tend to make indi- viduals of the same species differ from one another. (a) The direct influence of the environment, for ex- ample, more ample nourishment causing stronger growth.

(V) The indirect influence of the reproductive organs, which he assumes to be very sensitive to every kind of impression. (c) A change of habit. The domestic duck is forgetting how to fly, the ears of dogs hang as the result of disuse. In some cases a change in one quality will induce a change in another long-legged animals, for example, tend to have long heads ; organs of which the animal possesses more than one, rudimentary organs, and those of simple structure, show most variation.

The features distinguishing individuals show the greatest tendency to variation; those marking varieties are more constant, those characterizing orders show least variation. In another place the rule is formulated that the com- monest and most widely spread species are the most variable, and so are the species belonging to the large genera (i.e. in those orders which contain many genera, these genera have many species). Darwin calls these suggestions variously 'hypotheses', 'causes', 'rules', or sometimes 'laws' !

3. He terms the fact that offspring resemble their parents, heredity ; he shows that children inherit from their parents not only their common human characteristics, but their individual ones. Perhaps the correct way of viewing the whole subject would be to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly' (Origin of Species , p. 10). Thus he believed that the children of English work- men inherit large hands, that chewing coarse food has strengthened the facial muscles among primitive peoples, that the hard skin on the soles of the feet has been developed by walking. I

When he had collected his material concerning inherited characters, he arranged it so that it illustrated the follow- ing 'laws' : (a) If the parents acquire a character at a certain age, it will show itself in their offspring at about the same age. The chickens of various breeds of domestic fowl are closely similar but differentiate as they mature. (b) Periodicity is an inherited factor, e.g. the periodicity of the colour change in those animals which have a winter and a summer colour.

(c) The characters of the father are often inherited by his sons, those of the mother by her daughters, e.g. the horns of deer are only inherited by tire males. Some characters, however, can be passed on to both sexes, hence the male has a trace of mammary glands, organs which functionally are purely feminine. (d) Man artificially produces new varieties in the follow- ing way: When a deviation from the type occurs (as, for example, a large crop in a pigeon) the animal is used for breeding

purposes. The offspring inherit the deviation, some in greater degree than others. Those in whom it is most strongly marked are again used for breeding, and so, by artificial selection, a new variety of pigeon is produced the pouter pigeon. Taste and requirements determine the variety produced. The individual variations are the passive material which nature provides by 'chance', and from these man selects. The nature of the different types of dog, pigeon, rose, carnation, and all the other artificial varieties by which modern man has surrounded himself, cannot be understood merely by a comparative study of existing forms. The taste of the hunter, of the pigeon- fancier, of the gardener, &c. must be considered. )

(<?)(The Struggle for Existence. A bitch generally bears more puppies in a litter than she can adequately nourish. So her owner drowns some of them, and by the death of their brothers, the survivors flourish. Similar processes, Darwin said, are at work in Nature.) Plant and animal life is such that not all the offspring produced can survive. The increase is too rapid. A plant which produces two seeds annually would be represented by a million descen- dants at the end of twenty years, if all the seeds met with favourable conditions and survived. The elephant increases more slowly thanany other animal, nevertheless if we started with one pair of elephants, we should, seven hundred and fifty years later, have nineteen million individuals if all lived to old age and bore offspring at the normal rate.

Experience teaches us, Darwin continues, how field- mice, caterpillars, &c. increase when two favourable seasons follow one another. Nevertheless, the number of individuals in each species tends to remain constant, as large numbers of them disappear before maturity. Eggs and seeds are destroyed, the organisms die young or come to an untimely end, sometimes killed by unfavourable weather, sometimes by hunger, by predatory animals, by parasites and epidemics. This is what Darwin called the ^Struggle for Existence^ We need not assume that the fundamental cEaract eristic of life is a striving, an inner

urge, or* that the animals actually fight for life. It is circumstances that so order it that while some die early, others survive. And it is these natural external circum- stances which constitute the struggle for existence. (/) The result of this struggle is Natural Selection. Let us assume, says Darwin, that there is a fleet-footed animal inhabiting a district where wolves capture their prey by strength, agility, and swiftness. Wolves differ in individual qualities. The swifter will get food more easily than the less swift and so will increase the more rapidly. Hence the fleet of foot in each generation will survive, and a race of swifter wolves will replace their slower ancestors.

It is difficult to give any general rules for the way in which Natural Selection will act. Individual variations do not seem to follow any law. Many and various are the conditions which cause the destruction of one deviation, the maintenance of another. In general we can say that it is in the common species, containing many individuals and showing a great tendency to variation, that departures from the mean appear most readily, and such departures often survive. Further, the more ancient the species, the more varied become the districts it inhabits. Individual variations are the starting- point for new varieties new varieties lead on to new species, and so on. If natural selection continues to act for a longtime, changing conditions will give rise to new varieties and after still greater periods to new species, genera, &c. Thus, after millions of years, from one, or, at most, a few original forms, the whole catena of life has been evolved.

'Thus, from the war of nature, from famine and death, the most exalted objects which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is a grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one ; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved' (Origin of Species, p. 403).

Darwin suggests here, as in other places, that his idea is comparable with Newton's discovery of gravitation. As Newton discovered one law which all heavenly bodies obey, so Darwin discovered one law for all living things. Darwin indeed went further than Newton. The latter revealed one property common to the sun, the planets and all earthly bodies. The former explains, through the struggle for existence, not one but almost all the attributes which differentiate living organisms from one another. This great difference has been the cause of considerable con- fusion among Darwin's more ardent followers.

We have given the chief features of Darwin's theory, leaving out minor points which will be dealt with later. We may, however, mention a few of the deductions which he thought might be made from his theory. With the exception of a few general characteristics of living substance, which he did not define more closely, Darwin thought that every feature of the organism had developed by chance. Nothing had meaning in itself, nor would it have developed had not those special circum- stances which produced it existed. It is true that animals show a unity of plan. This uniformity, we now see, ex- presses their common origin. Homologous structures are inherited from a common ancestor. Analogous structures are adaptations to similar needs. The whole system of organic beings is merely a genealogical table, with the simplest organisms at its beginning, the most highly organized (which are to be regarded as the descendants of the simple forms) at its end. )

4. Some Peculiarities of Darwirfs Theory. The object of the new theory was to describe the events in Nature which give rise to new species, and so to under- stand the processes of Nature that we can relate the story of past ages, Darwin's aims were diametrically opposed to those of most of his predecessors. They endeavoured to find universal concepts which would include the multifarious activities of Nature ; he sought to obtain a clear idea of her

separate phenomena. Formerly reason was considered the only eternal reality appearances (phenomena) were but fleeting images of that reality. To Darwin, 'the way things happen 5 external events are everything ; reason and general ideas he absolutely neglects. He ignores every- thing that seems to deal with abstract ideas, with logic. He has no interest in the theories of his predecessors. He makes no attempt to analyse the teaching of Lamarck, of Erasmus Darwin, of Agassiz, of Kolliker, &c. it does not seem to have occurred to him that he should endeavour to understand them. He calmly writes that he does not understand the views of Cope (who was a disciple of Lamarck). In his works is hardly any discussion of the theories of others.

Darwin attacks the view of a species which was current in his day, since he believes it to be false ; but he never attempts any critical analysis of this erroneous idea. He does not even define his own concept, and it is character- istic of his whole attitude that the concept is not capable of definition. For it was just that fact which constituted his proof that there are no definite ideas, no fixed rules, in Nature. Try to discover Darwin's view on any general question ! Each body possesses certain capacities ; can these capaci- ties (the functions) affect the body (change it, for example), or are they merely a consequence of the bodily structure ? The two views had long been in dispute. What side does Darwin take in the controversy ? Which does he consider to be the cause and which the effect ?

'It is, however, difficult to decide, and immaterial for us, whether habits generally change first, and structure afterwards ; or whether slight modifications of structure lead to changed habits; both probably often occurring almost simultaneously' (Origin of Species, Has not this answer all the characteristics which made those of the Delphic Oracle famous ? relationships of animals, says Darwin ; only through it does a 'natural series' acquire any meaning. He conscientiously adds that minerals can also be arranged in a natural series ; but at that point he breaks off!

One asks why, if a genealogical tree is necessary to explain the series of living forms, is it not necessary to explain the mineral series. We know, for instance, that minerals and the elemental sub- stances can be thus arranged. In this case, there is, of course, no relation to genealogical succession, and no cause can at present be assigned for their falling into groups' (Origin of Species, p. 342). By this method Darwin makes any criticism of his teach- ing difficult. He explains the origin of species by natural selection, by artificial selection, by sudden changes, by correlated modifications, but he does not indicate the limits of any one of these explanations.

When Nageli pointed out to him that there are many forms pf life which cannot be explained by natural selection, Darwin modestly admitted that the principle could not have so universal an application as he had supposed. He never committed himself, never said how widely the idea could be applied, nor did he even state precisely in what cases it failed, so that this admission was only of academic value and did not influence the further development of his theory. (The Descent of Man, 1871, i, p. 152.)

Darwin objected to definitions because his whole thoughts were centered on the variety of Nature. When he should give a definition he overwhelms his readers with examples of that richness of Nature which seems to mock every definition. A species cannot be defined since so many transitional and irregular forms exist. We cannot define the 'struggle for existence', since so many different phenomena are included under this heading. The fight of a bird of prey with its victim, the mere maintenance of existence by a plant on the desert's edge, the too numerous seeds in some plants' fruits, the parasitism of the mistletoe

on the tree c in these several senses, which pass into each other, I use for convenience sake the general term of Struggle for Existence 3 (Origin of Species, p. 46). We must not forget that the word Species' is also used by him only for convenience. Try to learn something more definite from Darwin about natural selection. He will tell you that the inter- relationship between living organisms is so complicated that we cannot ever foresee how we may help the indi- vidual in its life-struggle.

'Throw up a handful of feathers, and all fall to the ground according to definite laws ; but how simple is the problem where each shall fall, compared to the action and reaction of the innumer- able plants and animals . . .' (Origin of Species, p. 54). He enjoys pointing out the tremendous interdependence of living organisms. His whole theory rests on the diversity of cause and effect. The partridge is grey because the hawk has good eyes. The rose smells sweet because the bee likes sweet scents. The amount of clover seed formed depends on the number of cats in the district, for cats hunt field-mice which destroy the nests of humble-bees. Humble-bees are necessary for the pollination of the clover, and so on. Since the interaction of events is so extremely complicated, Darwin can do no more than accumulate observations ; his works are full of quotations. He himself helped to make the inquiry into Nature's mani- fold mysteries exhaustive, for all through his life he was a great observer of natural phenomena.^

Darwin gave an excessive time to collecting suitable illustrations of his theory. He wanted to call his work on the Origin of Species 'Extracts from a Dissertation on the Origin of Species and Varieties by Natural Selection', and he only left out the word 'extracts' on the advice of his publisher ; as if a 400 page book was not enough to lay the foundation of his theory! His friends evidently did not think it enough, and we can constantly read that the book is only to be regarded as a 'first communication', and

that the actual demonstration of his theory is to be looked for in The Variation of Plants and Animals under Domestica- tion, which contains the whole of his material. The second part of his book on the 'Origin of Man' is almost unread- able because of the mass of quotations which it con- tains. Thus Darwin brought to its culmination that empirical point of view which had characterized English thought from the times of Bacon, Locke, and Hume, to that of Mill and Spencer. He and Mill agreed in another ex- tremely English idea. Mill was the mouthpiece of the teaching of his time, that morality has its foundations in utility ; moral behaviour is nothing more than behaviour useful to mankind; to die for your country is moral because it is useful to society, to the country.