202 GENERAL EVOLUTION". 



excessive force is evidently expended in the development of horns, 

 and complication of stomach and digestive organs. The excess 

 devoted to the latter region may account for the lack of teeth at 

 its anterior orifice, the mouth ; otherwise, there appears to be no 

 reason why the ruminating animals should not have the superior 

 incisors as well developed as in the odd-toed (Perissodactyl) Un- 

 gulates, many of which graze and browse. The loss to the osseous 

 system in the subtraction of digits may be made up in the develop- 

 ment of horns and horn-cores, the horn sheath being perhaps the 

 complement of the lost hoofs. It is not proposed to assert that 

 similar parts or organs are necessarily and in all groups comple- 

 mentary to each other. The horse has the bones of the feet still 

 further reduced than the ox, and is nevertheless without horns. 

 The expenditure of the complementary growth-force may be sought 

 elsewhere in this animal. The lateral digits of the Equidce are 

 successively retarded in their growth, their reduction being marked 

 in Hiypotlierium, the last of the three-toed horses ; it is accom- 

 panied by an almost coincident acceleration in the growth-nutri- 

 tion of the middle toe, which thus ajipears to be complementary 

 to them. 



The superior incisors of the Artiodadyla disa^Dpear coincident- 

 ally with the appearance of horns, which always exist in the tooth- 

 less division of the order, excej)t in some very small antelopes 

 (Cephalophus, etc.) where the whole amount of growth-force is 

 small. Possibly the superior incisors and horns are complementary 

 here. The retardation in development of the teeth in the higher 

 apes and men, as compared with the lower apes, is coincident with 

 the increase of number of brain convolutions. That this is not 

 necessarily coincident with reduction of teeth in other groups is 

 plainly proved by the rodents and CMromys, where the loss of many 

 teeth is complementary to the great size of the incisors of the mid- 

 dle pair. But in man there is no complementary increase of 

 other teeth, and the reduction is no doubt due to contraction of 

 the jaws, which is complementary to increase in other parts of the 

 cranium, in both apes and men. 



I am confident that the origin and loss of many structures may 

 be accounted for in this way, and the correlation of parts to each 

 other be measured accurately. 



Objection, — The first one which arises is that which the author 

 of the ^^ Vestiges of Creation " made against Lamarck's theory of a 

 similar kind, i. e., that by assuming that effort, use, and physical 



