416 VAKIETIES, INCIPIENT SPECIES. ' CHAP. xxi. 



species of a high class, a mammifer, for example, now passes, 

 may be expected to present us with a picture of the stages 

 through which, in the course of ages, that class of animals 

 has successively passed in advancing from a lower to a 

 higher grade. Hence the embryonic states exhibited one 

 after the other by the human individual bear a certain amount 

 of resemblance to those of the fish, reptile, and bird before 

 assuming those of the highest division of the vertebrata. 



Mr. Darwin, after making a laborious analysis of many 

 floras, found that those genera which are represented by a 

 large number of species contain a greater number of variable 

 species, relatively speaking, than the smaller genera, or those 

 less numerously represented. This fact he adduces in support 

 of his opinion that varieties are incipient species, for he ob 

 serves that the existence of the larger genera implies, in the 

 period immediately preceding our own, that the manufacturing 

 of species has been active, in which case we ought generally 

 to find the same forces still in full activity, more especially 

 as we have every reason to believe the process by which new 

 species are produced is a slow one.* 



Dr. Hooker tells us that he was long disposed to doubt 

 this result, as he was acquainted with so many variable small 

 genera, but after examining Mr. Darwin's data, he was com 

 pelled to acquiesce in his generalisation.! 



It is one of those conclusions, to verify which requires the 

 investigation of many thousands of species, and to which 

 exceptions may easily be adduced, both in the animal and 

 vegetable kingdoms, so that it will be long before we can 

 expect it to be thoroughly tested, and, if true, fairly appre 

 ciated. Among the most striking exceptions will be some 

 genera still large, but which are beginning to decrease, the 

 conditions which were favourable to their former predomi- 



* Origin of Species, ch. ii. p. 56. 



f Introductory Essay on Flora of Australia, p. vi. 



