RELATION OF ANIMAL MOTION TO ANIMAL EVOLUTION. 355 



convex surface, on a concave one which embraces it, and vice 

 versa. 



In the circulatory system, pressure has doubtless played an im- 

 portant part. Increased oxygenation of blood, the necessary con- 

 sequence of the purification of the atmosphere, would stimulate 

 the action of all the organs, including that of the heart. Greater 

 pressure on its walls and septa w^ould increase their size and 

 strength, and ultimately close such foramina as were not in the 

 course of the blood current, as the foramen septi ventriculorum 

 of reptiles, and the/, ovale. Increased force of the current would, 

 on the other hand, soon cause the enlargement of one or other of 

 the four or five pairs of primary aorta bows, and develop it at the 

 expense of the others, until finally the pre-eminence of one chan- 

 nel be secured and the aorta be the result. This part of the sub- 

 ject might be prolonged to an unlimited extent, but the above 

 illustrations must suffice to indicate the meaning of my proposi- 

 tions. 



III. 



That movements change the environment of a plant or an ani- 

 mal, or parts of them, is obvious enough. If we consider only 

 the reflex class, to which all the movements of plants and many 

 of those of animals belong, we perceive that but for them the or- 

 dinary functions of assimilation, circulation, etc., could not be 

 performed ; there would be no change in the contents of their 

 tubes and cells, and the environment of these would be unaltered. 

 But when we view the movements of the higher animals, w^e j)er- 

 ceive the immense importance of the powers and organs of move- 

 ment as a factor in evolution. It may be safely assumed that, 

 without powers of designed or adaptive movement, life would 

 never have advanced beyond the stage presented by the vegetable 

 kingdom. 



The stimuli which are effective in animal consciousness are 

 four, viz.: excessive temperature, hunger, danger from enemies, 

 and the reproductive instinct. These prompt to the movements 

 which we observe in animals in a wild state, and without which 

 it is evident that the animals themselves would soon cease to 

 exist. 



It can not be denied that organisms which are incapable of 

 moving from place to place in search of food, or of migration to 

 escape vicissitudes of temperature, are much more completely sub- 



