90 LECTUKES ON EVOLUTION 



III 



groups of animals and of plants may be, they 

 must all, at one time or other, have been con- 

 nected by gradational forms ; so that, from the 

 highest animals, whatever they may be, down to 

 the lowest speck of protoplasmic matter in which 

 life can be manifested, a series of gradations, 

 .Jeading from one end of the series to the other, 

 either exists or has existed. Undoubtedly that is 

 a necessary postulate of the doctrine of evolution. 

 But when we look upon living Nature as it is, we 

 find a totally different state of things. We find 

 that animals and plants fall into groups, the 

 different members of which are pretty closely 

 allied together, but which are separated by 

 definite, larger or smaller, breaks, from other 

 groups. In other words, no intermediate forms 

 which bridge over these gaps or intervals are, at 

 present, to be met with. 



To illustrate what I mean : Let me call your 

 attention to those vertebrate animals which are 

 most familiar to you, such as mammals, birds, and 

 reptiles. At the present day, these groups of 

 animals are perfectly well-defined from one 

 another. We know of no animal now living 

 which, in any sense, is intermediate between the 

 mammal and the bird, or between the bird and 

 the reptile ; but, on the contrary, there are many 

 very distinct anatomical peculiarities, well-defined ' 

 marks, by which the mammal is separated from 

 the bird, and the bird from the reptile. The 



