180 GENEPwAL EVOLUTION 



then the result of continued retardation, not of a new and adaptive 

 suppression, and is of direct systematic zoological value. 



In the incisors a reduction is also plainly visible, as we pass 

 from the most completely furnished mammals to the genus Homo. 

 One from the upper jaw is first lost, then, in the Cebidce, one from 

 the lower also. The number remains the same through the SimiidcB 

 and normal Hommidce, but, in the abnormal cases cited, the process 

 of reduction is continued and another incisor from each side disap- 

 pears. That this also is truly *^ retardation" is evident from the 

 fact, that the exterior incisor is the last developed, being delayed 

 in ordinary growth a year later than those of the inner pair. The 

 same retardation is seen in the quadrumane Cheiromys (the aye- 

 aye), and the whole order Rodentia. In the latter, the rare pres- 

 ence of the reduced second incisors, as in Lejjus, shows a less degree 

 of this modification. This retardation is also of systematic impor- 

 tance, and, should either of the characters described be constant in 

 any of the species of the genus Homo, would at once entitle it to 

 new generic rank. The very frequent absence of the posterior 

 molars (wisdom teeth) has been recently found to characterize a 

 race in India. Should this peculiarity prove constant, this race 

 would with propriety be referred to as a new genus of Hominidm, 

 as we have many cases of very similar species being referred to 

 different genera. It is altogether probable that such will, at some 

 future time, be the condition of some race or races of men.* 



I am now disposed to regard the above as the method of produc- 

 tion, not only of generic but of all other, including specific char- 

 acters. It would appear that, by excessive acceleration or retarda- 

 tion, some of the characters of a series may be skipped; but observa- 

 tions are not conclusive on this point, since very close examination 

 is necessary for the appreciation of very transitory embryonic con- 

 ditions. 



II. OK THE LAW OF REPETITIVE ADDITION. 



The origin of new structures, which distinguish one generation 

 from those which have preceded it, I have stated to take place 

 under the law of acceleration. As growth (creation) of parts usu- 

 ally ceases with maturity, it is entirely plain that the process of 

 acceleration is limited to the period of infancy and youth in all 

 animals. It is also plain that the question of growth is one of 



* The preceding section is merely an abbreviation, with new illustrations, of the 

 propositions brought forward in the writer's " Origin of Genera," 1868, where a con- 

 siderable extension of the subject will be found. 



