Chap. VII.] THEOEY OF NATURAL SELECTION. 317 



those advocated by Mr. Mivart, such as the sudden de- 

 velopment of the wings of birds or bats, or the sudden 

 conversion of a Hipparion into a horse, hardly any light 

 is thrown by the belief in abrupt modifications on the 

 deficiency of connecting links in our geological forma- 

 tions. But against the belief in such abrupt changes, 

 embryology enters a strong protest. It is notorious 

 that the wings of birds and bats, and the legs of horses 

 or other quadrupeds, are undistinguishable at an early 

 embryonic period, and that they become differentiated 

 by insensibly fine steps. Embryological resemblances 

 of all kinds can be accounted for, as we shall hereafter 

 see, by the progenitors of our existing species having 

 varied after, early youth, and having transmitted their 

 newly acquired characters to their offspring, at a corres- 

 ponding age. The embryo is thus left almost unaffected, 

 and serves as a record of the past condition of the species. 

 Hence it is that existing species during the early stages 

 of their development so often resemble ancient and 

 extinct forms belonging to the same class. On this 

 view of the meaning of embryological resemblances, and 

 indeed on any view, it is incredible that an animal 

 should have undergone such momentous and abrupt 

 transformations, as those above indicated; and yet should 

 not bear even a trace in its embryonic condition of any 

 sudden modification; every detail in its structure being 

 developed by insensibly fine steps. 



He who believes that some ancient form was trans- 

 formed suddenly through an internal force or tendency 

 into, for instance, one furnished with wings, will be almost 

 compelled to assume, in opposition to all analogy, that 

 many individuals varied simultaneously. It cannot be 

 denied that such abrupt and great changes of structure are 



