686 



DARWINIAN THEORY 



and thus those who admit the unity of domestic 

 races should be cautious in deriding the unity of 

 wild ones. 



Domestic races all exhibit adaptations to man's 

 use or fancy, rather than to their own good. The 

 key to this is man's power of selection : nature gives 

 successive variations, man accumulates these, so 

 making for himself useful breeds, and often, (e.g. 

 in sheep, cattle, roses, dahlias ) profoundly modifies 

 their character even in a single human lifetime ; so 

 that in all characters to which he attends, they 

 may differ more than the distinct species of the 

 same genera. Again, that unconscious selection 

 which results from every one trying to possess and 

 breed the best animals is yet more important than 

 conscious selection. Two flocks of Leicester sheep, 

 kept equally pure, appeared of quite different 

 varieties after fifty years. Such slowly accumu- 

 lated change explains why we know so little of the 

 origin of domestic races ; and its absence in regions 

 inhabited by uncivilised man explains why these 

 yield no plants worth immediate culture. Human 

 selection is facilitated ( 1 ) by the keeping of large 

 numbers, since variations will be more frequent, 

 and (2) by preventing free-intercrossing; some 

 species vary, however, more than others. 



Variation under Nature. All similar organisms 

 in nature present individual differences, more con- 

 siderable than is usually supposed. No two blades 

 of grass are alike, and far more marked differences 

 often occur, several castes or varieties sometimes 

 existing in the same sex. Between these castes, 

 and much more frequently between forms which 

 systematic botanists and zoologists rank as true 

 species, perfectly intermediate forms may occur. 

 No agreement about the definition of species (the 

 amount of difference necessary to give any two 

 forms specific rank ) has ever been come to ; thus, 

 in the British flora alone, there are nearly two 

 hundred disputed forms, and individual opinion is 

 in these cases the only criterion. As long as a 

 genus is imperfectly known, and its species founded 

 upon few specimens, they appear clearly limited. 

 But with fuller knowledge, intermediate forms 

 come in, and doubts as to specific limits augment. 

 The terms species and variety are thus arbitrarily 

 given to sets of individuals more or less closely 

 resembling each other. See VARIATION, SPECIES, 

 GENUS. 



Individual differences are thus of the highest 

 importance, as the first steps towards the slightest 

 varieties worth recording, these in turn towards 

 more distinct and permanent varieties, these varie- 

 ties again towards sub-species, and in the next 

 stage to species ; though extinction may often 

 arrest the process. 



The species which present most varieties are 

 those which have the greatest geographical range, or 

 the widest diffusion in their own territory, or which 

 possess the greatest number of individuals. In 

 the larger genera of each country the species vary 

 more frequently than in the smaller genera ; and 

 in many respects the species of large genera present 

 a strong analogy with varieties, which analogy is 

 alone intelligible on the view that they once 

 existed as mere varieties themselves. 



Struggle for Existence. All organic beings tend 

 to increase with extreme rapidity, so that if they 

 were not kept down, the earth would soon be 

 covered by the progeny of a single pair. This is 

 evidenced not merely by calculation, out by actual 

 observation of the extraordinary rapidity with 

 which plants and animals have spread, when in- 

 troduced into new and favourable circumstances 

 (e.g. thistles and rabbits into Australia). 



Since organisms then are reproducing themselves 

 so rapidly, and since all their offspring cannot 

 escape their enemies, get food, and live, much less 



leave progeny in turn since, in other words, the 

 doctrine of Malthus applies to animals and plants 

 with manifold force ( for these can have no artificial 

 increase of food, and no prudential restraints on re- 

 production ) there must in every case be a struggle 

 for existence, either of one individual with another 

 of the same species, or with the individuals of dis- 

 tinct species, or with the physical conditions of 

 life ; often, indeed, with all these at once, and 

 that more or less intensely throughout the whole of 

 life. 



The checks which prevent increase are most 

 obscure, and vary in each case. In all cases the 

 amount of food, of course, gives the extreme limit. 

 The youngest organisms generally sutler most ; 

 seedlings, for instance, are destroyed in vast num- 

 bers. Thus, even in a patch of ground purposely 

 dug and cleared, where no choking from other plants 

 could take place, 295 out of 357 seedling-weeds were 

 destroyed, chiefly by slugs and insects. So, too, 

 the stock of game on an estate depends chiefly upon 

 the destruction of vermin. Climate, however, is 

 highly important, and periodic seasons of extreme 

 cold and drought seem the most effective of all 

 checks a severe winter sometimes destroying four- 

 fifths or more of the birds of a locality. Epidemics, 

 too, may occur, especially where numbers have 

 inordinately increased. On the other hand, a 

 large number of individuals of the same species 

 is essential for its continued preservation. 



The complex relations of all animals and plants 

 to each other require illustration. The planting 

 of part of a heath with Scotch fir leads to a pro- 

 found alteration of its flora and fauna, while the 

 growth of these firs again is wholly dependent 

 upon the exclusion of cattle. Many flowers depend 

 for fertilisation on the visit of a special insect 

 e.g. red clover on humble-bees. But bees are 

 destroyed by field-mice, and consequently pro- 

 tected by cats ; hence, not only no bees, no clover, 

 but also the more cats, the 'more clover ! The 

 struggle for life is most severe between individuals 

 and varieties of the same species, and between 

 the species of the same genus, since these tend to 

 fill the same place in the economy of nature ; hence 

 we see the brown rat supplanting the black, and 

 the hive-bee supplanting its Australian congener. 

 The structure of every being is related to that of 

 the others with which it competes, or from which 

 it seeks to escape, or on which it preys ; as is alike 

 evident in the structure of the tiger and of the 

 parasite which clings to his hair. So, too, the 

 albumen of a seed is chiefly useful in favouring the 

 young plant's struggle for light and air against the 

 adult plants around. 



Natural Selection. But how will the struggle 

 for existence act with regard to variation ? Can 

 the principle of selection, so potent in the hands 

 of man, apply under nature? Most efficiently so. 

 Let us bear in mind ( I ) the constant occurrence 

 of variation ; ( 2 ) the infinite complexity of the 

 relations in which organisms stand to each other, 

 and to the physical conditions of life ; and con- 

 sequently (3) what infinitely varied diversities of 

 structure might be useful to each being under 

 changing conditions of life. Can it then be thought 

 improbable, seeing that variations useful to man 

 have undoubtedly occurred, that other variations 

 useful in some way to each combatant in the great 

 and complex battle of life should also occur in the 

 course of many generations ? And if such do 

 occur, can we doubt (remembering that many more 

 individuals are born than can possibly survive) 

 that individuals having some advantage, however 

 slight, have the best chance of surviving and of 

 reproducing their kind, while injurious variations 

 are destroyed ? This preservation of favourable 

 variations, and the destruction of injurious ones. 



