Claypole.] 130 [^^pHl 1, 



Now conceive an organism as tlie moving body, tlie motion of the latter 

 being represented by the specific life of the former. As the moving body 

 travels through space, so the organism travels thi-ough time. If unaf- 

 fected by outside disturbing force the former continues in a straight line. 

 So if unaffected by changes in its environment, the latter remains unva- 

 ried from generation to generation, merely changing its position in time as 

 the former in space. Thus each would continue indefinitely, the same in 

 all respects except position, after many thousand years had passed away. 

 But on its way the flying body approaches some other mass of matter, 

 and immediately feels an influence by which its own motion is modified. 

 So on its way through time the organism comes into a different environ- 

 ment to the influence of which it responds by modification of structure or 

 habits or both. These modifications are the necessary consequences of the 

 changes in its surroundings. In the former case we call them physical, in 

 the latter natural. Hut in neither do we know anything of the mode of 

 working. Of the "why" and the "wherefore" of both we are equally 

 ignorant. The mechanician sometimes imagines that by attributing the 

 one to " universal gravitation " he has explained it. But he has not. He 

 knows nothing of the nature or of the cause of this universal gravitation. 

 The biologist is not yet sufficiently advanced to generalize variation and 

 give a definite name to its cause. But with this unimportant difference 

 the two are in the same predicament. 



The parallel may be followed a step farther. The body moving through 

 space and the organism through time are alike in another respect. The 

 former maj^ be drawn forward and its motion accelerated by an outside 

 force. So the latter may vary and improve under the influence of envi- 

 ronment. The former may be retarded and its motion may be diminished 

 or destroyed. So by unfavorable environment the latter may vary 

 in a prejudicial manner until extinction ensues. Yet again the for- 

 mer may, under the influence of the disturbing forces, change its direction 

 without either acceleration or retardation. So the latter may vary in 

 directions which shall be perfectly neutral in their effect upon its welfare, 

 and the new form may be as capable of survival as the old. In both cases 

 the variable is perfectly passive and plastic in the hands of its environ- 

 ment, and the environment is perfectly indifferent to the welfare of the 

 variably. The ensuing variate is an outcome of the conditions of the 

 world around it and must take its chance among them, living if in har- 

 mony, or dying if in discord. 



In both the above cases the crucial experiment is beyond our reach. To 

 obtain absolutely uniform rectilinear motion there must be only one body 

 in the universe, and no resisting medium. To obtain an absolutely 

 unchanging variable organism there must be no alteration of the condi- 

 tions of existence. Both are alike unattainable. Yet, as with the mechani- 

 cian so with the biologist, every approximation brings him nearer to the 

 desired result. The vertebrates are least capable of enduring changes of 

 environment, and land surfaces afford the most variable conditions of life. 



