THEORY OK NATURAL SELECTION. 226 



tinct species, that numerous and wonderfully fine grada- 

 tions can be traced, connecting together widely different 

 structures. 



Many large groups of facts are intelligible only on the 

 principle that species have been evolved by very small 

 steps. For instance, the fact that the species included in 

 the larger genera are more closely related to each other, and 

 present a greater number of varieties, than do the species 

 in the smaller genera. The former are also grouped in 

 little clusters, like varieties round species, and they pre- 

 sent 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 

 tacts, all pointing in the same direction, could be added. 



Although very many species have almost certainly been 

 produced 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 favor of 

 this view, such as the sudden crystallization 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, sup- 

 ports at first sight the belief in abrupt development. But 

 the value of this evidence depends entirely on the perfec- 

 tion of the geological record, in relation to periods remote 

 in the history of the world. If the record is as frag- 

 mentary as many geologists strenuously assert, there is 

 nothing strange in new forms appearing 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 connect- 

 ing links in our geological formations. But against the 

 belief in such abrupt changes, embryology enters a strong 



