34 ANNUAL OF SCIENTIFIC DISCOVERY. 



given to those gradual changes in the conditions of a country affecting the 

 due supply of sustenance to animals in a state of nature. I have also 

 pointed out the characters in the animals themselves calculated to render 

 them most obnoxious to such extirpating influences : and on one occasion I 

 have applied the remarks to the explanation of so many of the larger species 

 of particular groups of animals having become extinct, whilst smaller 

 species of equal antiquity have remained. In proportion to its bulk is the 

 difficulty of the contest, which, as a living organized whole, the individual 

 of such species has to maintain against the surrounding agencies that arc 

 ever tending to dissolve the vital bond and subjugate the living matter to 

 the ordinary chemical and physical forces. Any changes, therefore, in such 

 external agencies as a species may have been originally adapted to exist in, 

 will militate against that existence in a degree proportionate, perhaps in a 

 geometrical ratio, to the bulk of the species. If a dry season be gradually 

 prolonged, the large mammal will suffer from the drought sooner than the 

 small one ; if such alteration of climate affect the quantity of vegetable 

 food, the bulky herbivore will first feel the effects of stinted nourishment ; 

 if new enemies are introduced, the large and conspicuous quadruped or bird 

 will fall a prey, while the smaller species conceal themselves and escape. 

 Smaller animals are usually also more prolific than larger ones. " The 

 actual presence, therefore, of small species of animals in countries where 

 larger species of the same natural families formerly existed, is not the con- 

 sequence of any gradual diminution of the size of such species, but is the 

 result of circumstances which may be illustrated by the fable of the ' Oak 

 and the Reed ; ' the smaller and feebler animals have bent and accommo- 

 dated themselves to changes which have destroyed the larger species." 

 No doubt the type-form of any species is that which is best adapted to the 

 conditions under which such species at the time exists ; and as long as those 

 conditions remain unchanged, so long will the type remain; all varieties 

 departing therefrom being in the same ratio less adapted to the environing 

 conditions of existence. But, if those conditions change, then the variety 

 of the species at an antecedent date and state of things will become the 

 type-form of the species at a later date, and in an altered state of things. 

 Observation of animals in a state of nature is required to show their degree 

 of plasticity, or the extent to which varieties do arise, whereby grounds may 

 be had for judging of the probability of the elastic ligaments and joint- 

 structures of a feline foot, for example, being superinduced upon the more 

 simple structure of the toe with the non-retractile claw, according to the 

 principle of a succession of varieties in time. Observation of fossil remains 

 is also still needed to make known the antetypes, in which varieties, analo- 

 gous to the observed ones in existing species, might have occurred, so as to 

 give rise ultimately to such extreme forms as the Giraffe, for example. 

 The aboriginal laws of the geographical distribution of plants and animals 

 have been modified from of old by geological and the concomitant climatnl 

 changes ; but they have been much more disturbed by man since his intro- 

 duction upon the globe. The serviceable plants and animals which he has 

 carried with him in his migrations have flourished and multiplied in lands 

 the most remote from the habitats of the aboriginal species. Man has, also, 

 been the most potent and intelligible cause of the extirpation of species 

 within historic times. He alone, with one of the beasts which he has 

 domesticated the dog is cosmopolitan. The human species is repre- 

 sented by a few well-marked varieties; and there is a certain amount of cor- 



