32 INITIATIVE IN EVOLUTION 



is not the same as that on the opposite side, renders their environ- 

 ments discontinuous, or different from that of another amoeba 

 occupying a position and " environment " which we should consider 

 identical. And this consideration applies to the other few 

 " tropisms " which enter into their little lives. This statement 

 may be difficult to prove, but it is a necessity of thought. An 

 illustration may assist one in visualising such discontinuity. A 

 fly is seen crawling at its own pace up one of the great pillars of 

 St. Paul's Cathedral. It comes to one of the thin layers of cement 

 worn down with age and so delicate that a man can just see it in 

 a good light. The fiy pauses, and passes into what is for it a chasm, 

 with as much relative deliberation as the man would show in 

 passing across a deep railway cutting. The number of pictures 

 that could be made of cases corresponding to that of the amoeba is 

 incalculable. A few will suffice. Two plants of the common nettle 

 are growing on the south side of a ditch in a lane, one rooted a foot 

 higher than the other. The upper one receives throughout its 

 life from wind and sun stimuli slightly different from those received 

 by the lower, and from the soil slight!}' less moisture. These 

 again receive stimuli very different from another pair on the northern 

 side of the lane. Again in windy weather a clump of sycamores 

 facing the south-west in England, and situated on the ridge of an 

 eminence, will receive very different stimuli from a similar clump 

 on the north-eastern slope of this eminence, and will demonstrate 

 the fact, as to force of wind, by a marked slope to the North East. 

 Even in either of the clumps the individual trees present varying 

 degrees of slope according to their position. The total experience 

 of these two clumps of sycamores and of any two in each clump 

 is obviously different. In a windy situation you can tell in July 

 which is the prevailing wind by noting the main inclination of the 

 ears of corn in a field. Again two male sticklebacks in a pond will 

 make nests for the eggs, there to be deposited, and one will choose 

 a spot on the southern and another on the northern side of a little 

 promontory of soil and stones at the edge of the pond. One will 

 find ready for him materials for building his nest different from 

 those of his rival, and he and his wife and family will receive for 

 that season very different stimuli, and so will the stimuli differ 

 in other phases of their existence in a pond occupying a few square 

 j^ards. On a sandy bank in a garden facing south you may discover 

 two little caves ingeniously hidden by a small opening, and in each 

 of them you can see a toad. Though these are only a few feet 

 apart one is more widely open to sun and -wind than the other and 

 one deej>er than the other, and whatever the other activities of the 



