438 NATURE AND MAN. 



has been the effect of &quot; physical causes,&quot; when the most essential 

 factor in that entire &quot;aggregate of antecedents,&quot; which (according 

 to J. S. Mill) constitutes the &quot; cause,&quot; is the &quot; unknown quantity &quot; 

 which we designate as the &quot; constitution &quot; of the organism itself. 

 As I have already pointed out, we do not get any nearer to the 

 explanation of this constitution by tracing it backwards ancestrally; 

 for supposing Itosa, Rubus, Salix, and Brassica to have derived 

 their respective peculiarities by &quot; natural selection &quot; from among 

 previous varieties, the question recurs, Whence those varietal 

 modifications? No physical agencies can be assigned, at any 

 stage whatever of the descent, as an adequate account of them ; 

 since, for those agencies to take effect, there must have been a 

 concurrent capacity for variation, either in the organism itself, or in 

 its germ, in virtue of which its varietal forms were engendered. The 

 necessity for this factor is evinced by the negative results of its 

 deficiency, shown in the &quot;rareness&quot; of many wild plants, and the 

 unconquerable resistance made by others to all improvement by 

 cultivation. 



Precisely the same thing obtains in the Animal kingdom. The 

 lowest Protozoa, of which Amoeba is the type, find in every pond 

 the organic materials which they require for their sustenance ; and 

 live and multiply under all ordinary ranges of temperature. But 

 most Animals of high organization require particular kinds of 

 food : some being purely carnivorous, others purely herbivorous ; 

 whilst others, like Man, are omnivorous, and are thereby enabled 

 to sustain themselves on a greater variety of alimentary sub 

 stances. So, again, all the higher types of Animals need an 

 elevated temperature for the maintenance of their activity ; but 

 while the &quot;cold-blooded,&quot; as Insects and Reptiles, are entirely 

 dependent upon the temperature of the medium they inhabit, and 

 are therefore reduced to a state of torpidity by its depression, 

 &quot; warm-blooded &quot; Birds and Mammals carry their heating-furnaces 

 about with them, and are thus in a great degree independent of 

 depressions in external temperature. Yet even with this advan 

 tage, we find the whole Quadrumanous order and the larger 

 Carnivora, as well as the (existing) Elephant, Rhinoceros, and 

 Hippopotamus, restricted to tropical or sub-tropical climates ; none 

 of them being able to resist the winter cold of the temperate zone. 



