206 THE ORIGIN OF SPECIES 



species, instead of to distinct species, that numerous and wonder- 

 fully fine gradations 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 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 charac- 

 ters; 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 be 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 dif- 

 ferent 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 sub- 

 stances, 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 en- 

 tirely on the perfection 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 advo- 

 cated 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 modifica- 

 tions 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 undistiiv- 

 guishable at an early embryonic period, and that they become 

 differentiated by insensibly fine steps. Embryological resem- 



