284 ZOOLOGY. 



brought about? This is a question which has engaged 

 the attention of many of the ablest biologists from the 

 time of Darwin to the present day, and the problems 

 which it involves are so complicated that only a brief 

 outline of the present state of our knowledge of the 

 subject can be given here. 



We may point out in the first place that Darwin's 

 theory of Natural Selection, though proposed as an 

 explanation of the origin of species, was in reality an 

 explanation of the origin of adaptations. It was as- 

 sumed by Darwin that species differed from one another 

 because they were adapted to different modes of life. 

 The marvellous adaptations that occur in animals and 

 plants are obvious to every naturalist : we may take 

 the structure of a bird, or of a whale, or of a flat-fish 

 as conspicuous examples. On the other hand systema- 

 tists spend the greater part of their lives in studying 

 and describing the minute differences which distinguish 

 allied species from one another, and in the great majority 

 of cases no evidence has been discovered that these 

 differences correspond to differences in mode of life, or 

 have any function or utility which could bring them 

 within the scope of the theory of Natural Selection. 



There are then two distinct problems to be considered, 

 the Qrigin^of^ species and the origin of adaptations in 

 other words, the^auses by which organisms are so modi- 

 fied as to be fitted for their modes of life, and the causes 

 by which organisms living under similar conditions are 

 divided into distinct species. It may be in some cases, 

 as in that of the rabbit and hare, that the differences 

 of species correspond to differences of habit, but it is 

 certain that this is not the case with a vast number of 

 specific characters, and these have to be explained. 



7. Gametes and Heredity. Weissman by his theory 

 of the independence of the germ-plasm directed the atten- 

 tion of biologists to the reproductive cells as the only 

 connecting links between parents and offspring. Without 

 entering into the theoretical question of the existence of 

 a special germ-plasm distinct from the ordinary proto- 



