The science of life : fully illustrated in tone and line and including many diagrams

BOOK 4

not hamper the operation of Natural Selection in the very least that he should do this. But Natural Selection has been no more “exploded” by recent research than the rejection of under-weight coins at the Mint has been exploded by the doctrine of relativity. Wherever there are favourable or unfavourable hereditable variations Natural Selection must be at work.

Nearly three-quarters of a century have passed since the controversial cataclysms of the mid-Victorian period, and Darwinism has been criticized in every conceivable way. It cannot be said that it has been destroyed, but it has undergone restatement in certain respects.

‘The modification of a species by the natural selection of variations is still an undefeated theory. That idea from Darwin’s writings lives and flourishes. The remoulding of Darwinism has concerned the part of it which deals with the mechanism of heredity and the intimate nature of variations. For in Darwin’s time hardly anything was definitely known about theinheritance of individual differences. The chromosomes to be presently described had not yet been seen ; the essential facts of fertilization were unknown ; most important of all, experimental breeding had not drawn a clear distinction between variations which are inherited and those which are not.

Since that time accurate knowledge has accumulated on these questions. The microscopic changes in the germ-cells that accompany fertilization were observed; the chromosomes, the bearers of the physical basis of heredity, were discovered, and their complicated but regular movements were traced as they passed from one generation to the next. Moreover, the study of inheritance was attacked experimentally. Even in Darwin’s time the Austrian Abbé Mendel (1822-1884) had experimented on the interbreeding of varieties of plants and had discovered the two most fundamental laws that govern hereditary transmission. But the significance of his work was not recognized at the time. His communication to the little Natural History Society at Brunn (now called Brno) dealt chiefly with peas and arithmetic, not the sort of things that cause excitement and clamour, and in the confused tumult of the nineteenth-century Evolution controversy they passed unnoticed. Only in the opening years of the twentieth century was his work disinterred and brought to bear on the discussion of Evolution.

This rediscovery was the stimulus for an enormous amount of careful experimental

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CHAPTER 1

breeding of animals and plants. New conceptions arose of how variations originate and are handed on from generation to generation, and of how they may play their part in the struggle for existence.

Here we are merely revising the broad issues before us. Later on we shall discuss the question whether variations occur through a “ life-force’ driving the whole species in a definite direction, or through some blind disposition to vary evoked by the action of external conditions upon the reproductive process. ‘That is an interesting issue of profound importance. And we shall have to consider the still-vexed question whether the inheritance of individually acquired characteristics, which was the essence of the original theory of Lamarck, occurs or no. This was flatly denied by Weismann (18341914) and disproved in many instances. He and his followers are sometimes spoken of as Neo-Darwinians. They believe variation to be a purely random process, resulting neither from a persistent urge nor from the Lamarckian moulding of the individual body; the direction of Evolution being determined entirely by Natural Selection. Their complete denial of the evolutionary value of individual experience gives a flavour of hard predestination to their views. Unless it vary by the grace of unknown forces, a struggling lineage is doomed ; no individual luck or effort can save it. And clearly no education, no social protection can cure the innate defects of any inferior human family. Tts réle is extinction.

A very fine and curious issue to which we shall later direct the reader’s attention is that of Orthogenesis. It is alleged that in a certain number of cases species, even though fairly well adapted to their conditions and without experiencing any change of conditions, have by virtue of a sort of inner drive, an innate destiny of the species, gone through considerable evolutionary change. Professor Henry Fairfield Osborn finds this drive convincingly displayed in various fossil series, such as the horses, camels, and titanotheres. Of course, where the changes produced by the orthogenesis have been disadvantageous, Natural Selection has at last arrested the drive and extinguished the line. But the supposition that the new form, new structure or other characteristic is not advantageous or is insufficiently advantageous to have “ survival value” is difficult to establish. It may involve an advantage which the observer has not recognized. And it is still more difficult to show that the drive towards variation in a definite direction was innate.