24 THE MODERN STUDY OF ZOOLOGY 



types, but that each type had its own mode of 

 development, and that all the animals included in 

 any one type agreed with one another in the funda- 

 mental features of their development. Thus the 

 several members of the type of animals known as 

 Vertebrata all develop in a fundamentally similar 

 manner ; in their earlier stages they resemble one 

 another so closely that it is often no easy matter 

 to distinguish one from another, the characteristic 

 differences not appearing till the later stages of 

 development. A good example of this is given by 

 von Baer himself, as follows : " In my possession 

 are two little embryos in spirit, whose names I 

 have omitted to attach, and at present I am quite 

 unable to say to what class they belong. They 

 may be lizards, or small birds, or very young mam- 

 mals, so complete is the similarity in mode of 

 formation of the head and trunk in these animals. 

 The extremities, however, are still absent in these 

 embryos ; but even if they had existed in the 

 earliest stages of their development we should learn 

 nothing, for the feet of lizards and mammals, the 

 wings and feet of birds, no less than the hands and 

 feet of men, all arise from the same fundamental 

 form." 



As soon as the idea of the mutability of species 

 was fairly grasped, the reason of these embryo- 

 logical resemblances became apparent, and it was 

 seen^that if two species or groups of animals develop 

 in the same way this is due to their being geneti- 

 cally allied to one another. 



Further consideration of this question led ulti- 



