PROFESSOR HUXLEY'S LECTURES. 213 



Had that constitution been other than what it was, the bones 

 would have been dissolved, the beds of sandstone would have fallen 

 together, become one mass, and not the slightest indication that the 

 animal had existed would have been discovered. 



I know of no more striking evidence than this fact affords, of the 

 caution which should be used in drawing the conclusion, from the 

 absence of organic remains in a deposit, that animals did not exist at 

 the time it was formed. I believe that, having the right understand- 

 ing of the doctrine of evolution on the one hand, and having a just 

 estimation of the importance of the imperfection of the geological 

 record on the other, all difficulty from the kind of evidence to which 

 I have adverted is removed; and we are justified in believing that 

 all such cases are examples of what I have designated negative 

 or indifferent evidence that is to say, they in no way directly ad- 

 vance the theory of evolution, but they are no obstacle in the way 

 of our belief in the doctrine. 



I now pass on to the consideration of those cases which are not 

 for reasons which I will point out to you by-and-by demonstrative of 

 the truth of evolution, but which are such as must exist if evolution 

 be true, and which therefore are, upon the whole, strongly in favor of 

 the doctrine. If the doctrin-e of evolution be true, it follows that, how- 

 ever diverse the different groups of animals and of plants may be, 

 they must have all, at one time or other, been connected by gra- 

 dational forms ; so that, from the highest animals, whatever they 

 may be, down to the lowest speck of gelatinous matter in which life 

 can be manifested, there must be a sure and progressive body of evi- 

 dence a series of gradations by which you could pass from one end 

 of the series to the other. Undoubtedly that is a necessary postulate 

 of the doctrine of evolution. But, when we look upon animated Na- 

 ture as it at present exists, we find something totally different from 

 this. We find that animals and plants fall into groups, the different 

 members of which are pretty closely allied together, but which are 

 separated by great breaks or intervals from other groups. And we 

 cannot at present find any intermediate forms which bridge over these 

 gaps or intervals. To illustrate what I mean: Let me call your at- 

 tention to those vertebrate animals which are most familiar to you, 

 such as mammals, and 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 reptile; 

 but, on the contrary, there are actually some very distinct and ana- 

 tomical peculiarities, well-defined marks, by which the mammal is 

 sepai-ated from the bird, and the bird from the reptile. The dis- 

 tinctions are apparent and striking if you compare the definitions 

 of these great groups as they now exist. At the pi-esent day there 

 are numerous forms of what we may call broadly the pig tribe, 



