REVIEW OF THE MODERN DOCTRINE OF EVOLUTION. 227 



display energy in a number of radiating lines rather than in the 

 spaces between them ; and in longitudinal animals, a longitudinal 

 axis exceeds all others in extent and importance. In the highest 

 animals its results are much more evident at one extremity of the 

 axis (head) than at the other, and the diyerging lines are reduced 

 to four (the limbs). In each species the movements of this force 

 are uniform and habitual, and it is evident that the habit is so 

 deeply seated that only a very strong dynamic interference can 

 modify or divert it. The interfering forces are probably all those 

 transmissible through living tissue, and especially molar force. 

 Thus every species has its own specific kind of bathmic force. 



The characters of living beings are either adaptive or non- 

 adaptive ; they are either machines especially fitted to meet the 

 peculiarities of their environment, or they are not. Among the 

 latter may be ranged rudimental structures and also many others 

 of no sufficient use. They are all due either to excess or defect of 

 growth-force ; they are either consequences of a removal of nutri- 

 tive material to other portions of the body ; or they are due to an 

 excess of such material which renders an organ or part useless 

 through disproportionate size. Of the former class may be cited 

 the absence of the tail in some monkeys and birds ; also of the 

 teeth in some Cetaceans ; of the latter kind are the enormous 

 tusks of the mammoth and the recurved superior canines of the 

 bab}Tussa. The change of destination of this material has been 

 probably due to the construction of adaptive machines whose per- 

 fection from time to time has required the use of larger and larger 

 proportions of force and material. 



In considering the origin of adaptive structures, two alterna- 

 tive propositions are presented to us. Did the occasion for its use 

 follow the appearance of the structure, or did the need for the 

 structure precede its appearance ? The following answer to the 

 question has always been the most intelligible to me. Animals 

 and plants are dependent for existence on their environment. It 

 is an every-day experience that changes in environment occur 

 without any preparation for them on the part of living things. If 

 the changes are very great, death is the result. It is evident that 

 the influence of environment is brought to bear on life as it is, or 

 has been, and that special adaptations to it on their part must fol- 

 low, not precede, changes of climate, topography, population, etc. 

 We have another important consideration to add to this one, viz., 

 the well-known influence of use, i. e., motion, on nutrition. Ex- 



