248 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



spend on one side, nature is forced to economize on 

 the other side." I have expressed the same view in 

 the following language : ^ 



" The complejuetitary dimmution of growth-nutrition 

 follows the excess of the same in a new locality or or- 

 gan, of necessity, if the whole amount of which an 

 animal is capable be, as I believe, fixed. In this way 

 are explained the cases of retardation of character seen 

 in most higher types. The discovery of truly comple- 

 mentary parts is a matter of nice observation and ex- 

 periment." An apparent illustration is that of the in- 

 crease of the median digits in the diplarthrous Ungu- 

 lata contemporaneously with the diminution and atro- 

 phy of the lateral digits. This is, however, an exam- 

 ple of the relative effects of use and disuse, which pro- 

 ceed contemporaneously, and it is probable that most 

 if not all cases of complementary growth-relations may 

 be expressed in this way. Thus the orthognathism of 

 the higher human races is accompanied by full frontal 

 development, the two modifications constituting a re- 

 tardation of the postembryonic growth of the face. 

 But this change can be traced to use, increased brain 

 action enlarging that organ, and expanding its osseous 

 case, probably at the expense of lime salts which would 

 otherwise go to the jaws. Reduction of teeth in Ar- 

 tiodactyla and in man cannot be regarded as a useful 

 character in itself, but it is complementary to the de- 

 velopment of other characters which are useful. 



Under the same head may come perhaps, the facts 

 included by Mr. Darwin under the head of " Correlated 

 Variation." Of these he says, "I mean by this ex- 

 pression that the whole organization is so tied together 



1 " Method of Creation, " Proceedings of the American Pliilosophical Society, 

 1871, p, 257; Origin of the Fittest, 1887. p. 201. 



