96 TEE ZOOLOGIST. 



admit that matters may appear somewhat complicated. So 

 diverse, indeed, do all these examples of hybridization appear to 

 most writers in relation to the usual classification, that I am not 

 aware that any recent authority has made any endeavour to put 

 forward any theory on these matters. The best they seem to 

 do, that is, the few who comment thereon, is to make some such 

 statement as " we now know that hybrids are not by any means 

 invariably infertile," while many others give no details or 

 examples of such results as I have given above, nor even refer 

 to the subject at all. Dewar & Finn * and Bartlett f certainly 

 produce an array of instances, but apparently make no attempt 

 to summarise or explain them, and they therefore leave the 

 subject much the same as when they entered upon it. 



Other writers touching on these matters explain that, 

 whereas fertility was formerly considered to be the absolute 

 test of a species, later investigations have discountenanced 

 this theory. Such criticism is merely negative, and is very 

 possibly far more unsound than the original belief which it 

 condemns. 



Many modern biologists are fond of drawing attention to the 

 inviolability of the germ-plasm, which they explain is housed, 

 guarded, and handed on from generation to generation, unin- 

 fluenced by and unaffected by all " indefinite variations," which 

 are therefore not inheritable. To a certain extent only does this 

 appear feasible. It seems altogether too strict and unplastic a 

 way of putting matters, nor does it appear sound reasoning com- 

 pared with Darwin's statement that environment and change of 

 diet are the causes of evolution and differentiation of species. 

 That these act on the germ-plasm seems probable, affecting it 

 through the digestion and blood and inducing it, as the parent 

 of the next generation (plasmatic and corporeal) to, as it were, 

 initiate and stimulate these variations in the latter, to be in 

 turn (the next generation) acted on in the same manner and 

 urged further along the new path. 



The seeming resultant that strikes one as produced by all 

 this is a belief to a certain extent only in the immutabilty of 

 the germ-plasm, in that it is not so vitally affected as the out- 

 ward form, as shown by the " fluctuations" and "mutations" 

 :: ' The Making of Species.' f 'Wild Animals in Captivity.' 



