Chap. VII.] THEORY OF NATURAL SELECTION. 203 



grouped in little clusters, like varieties round species; and they 

 present other analogies -with varieties, as was shown in our second 

 chapter. On this same principle we can understand how it is that 

 specific characters are more variable than generic characters ; and 

 how the parts which are developed in an extraordinary degree 

 or manner are more variable than other parts of the same species. 

 Many analogous facts, all pointing in the same direction, could bs 

 added. 



Although very many species have almost certainly been pro- 

 duced by steps not greater than those separating fine varieties ; yet 

 it may be maintained that some have been developed in a different 

 and abrupt manner. Such an admission, however, ought not to be 

 made without strong evidence being assigned. The vague and in 

 some respects false analogies, as they have been shown to be by 

 Mr. Chauncey Wright, which have been advanced in favour of this 

 view, such as the sudden crystallisation of inorganic substances, or 

 the falling of a facetted spheroid from one facet to another, hardly 

 deserve consideration. One class of facts, however, namely, the 

 sudden appearance of new and distinct forms of life in our geological 

 formations supports at first sight the belief in abrupt development. 

 But the value of this evidence depends entirely on the perfection oi 

 the geological record, in relation to periods remote in the history 

 of the world. If the record is as fragmentary as many geologists 

 strenuously assert, there is nothing strange in new forms appeal- 

 ing as if suddenly developed. 



Unless we admit transformations as prodigious as those advocated 

 by Mr. Mivart, such as the sudden development 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 formations. 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 em- 

 bryonic period, and that they become differentiated by insensibly 

 fine steps. Embryological resemblances of all kinds can be ac- 

 counted for, as we shall hereafter see, by the progenitors of our 

 existing species having varied after early youth, and having trans- 

 mitted their newly acquired characters to their offspring, at a 

 corresponding age. The embryo is thus left almost unaffected, aud 

 serves as a record of the past condition of the species. Hence it 

 is that existing species during the early stages of their develop- 

 ment so often resemble ancient and extinct forms belonging to the 

 same class. On this view of the meaning of embryological rcsein- 



