466 



ELEMENTARY BIOLOGY 



the world. In Germany there has been found a remarkably 

 complete series showing successively different types of snails 

 leading down to the present-day forms. Without regard to the 

 question as to how it comes about that descendants do differ 

 from their ancestors, there can be but one reasonable explana- 

 tion of the facts, — namely, that there has been modification 

 of plants and animals in the course of their descent. 



Protorohippus 

 Eocene 



Mesohippus 

 Oligocene 



Protohippus 

 Miocene 



Equus 

 Modem 



Fig. 249. Evolution of the horse 



Our ideas of the probable ancestors of modern animals are based on fossil remains. 

 These represent real organisms that lived thousands and thousands of years ago. In 

 the diagram the oldest type does not resemble the familiar horse very strikingly, but 

 with each succeeding age we find animals having a closer resemblance to the horse of 



to-day. (After Osborn) 



489. Evidence of evolution from structures of organisms. 



We have observed over and over again that individual plants 

 and animals- differ from each other, and that at the same time 

 they resemble each other in groups. The members of a group 

 that are sufficiently alike to be recognized by the casual observer 

 we speak of as being of the "' same kind." Thus, while no two 

 oak trees are exactly alike, they are all sufficiently alike to be 

 recognized as of the same kind. Now we expect, from our 

 observations, that all the oak trees, however different they 

 may be from each other, will give rise to new plants that will 

 also be enough like the parents to be classed as oaks. And 

 for the same reasons we take it for granted that all the oaks 

 of to-day are related, in the same sense in which we speak 

 of our cousins and second cousins as related. That is to say, 



