SPONTANEOUS GENERATION AND EVOLUTION : CONCLUSION 387 



many directions, but he will always select the one path which offers 

 him the best prospects of a comfortable settlement, and will follow it 

 only to the nearest suitable place of abode, and never further. The 

 transformation of a species only goes on until it has again completely 

 adapted itself. In this way he will in the course of years have 

 traversed a large number of different places which, taken together, 

 may lie in a strange and unintelligible course, but this course has 

 nevertheless not arisen through a mere whim, but through the two- 

 fold necessity of starting from a given spot — that in which he had 

 previously lived — the constitution of the species, and secondly of 

 choosing the most promising among the many available paths. 



But chance does play a part in determining the route of the 

 traveller, for on it depends the nature of the conditions in the 

 surroundings of his previous dwelling-place, when he is forced to 

 make another move ; for these conditions change, colonies are ex- 

 tended or depopulated, a town previously cheap becomes dear, 

 competition increases or decreases, disease breaks out or disappears ; 

 in short, the chances of a pleasureable sojourn in a particular place 

 may alter and determine the wanderer who is on the point of leaving 

 his place of abode to take a different direction from that which he 

 would probably have chosen, say, ten years earlier. 



The analogy might be carried further, as, for instance, to illustrate 

 the possibility of a splitting up of the species; we may suppose 

 that instead of one wanderer there is a pair, who found a family 

 at their first halting-place. Children and grandchildren grow up in 

 numbers and food becomes scarce. One part of the descendants still 

 finds enough to live upon, but the rest set out to look for a new 

 habitation. In this case, too, many paths, sidewards or backwards, 

 stand open to the wanderers, but only those paths will be actually 

 and successfully followed by any company of them which will lead to 

 a habitable place where settlement is possible. If some of the descen- 

 dants follow paths with no such prospect they will soon turn back 

 or will succumb to the perils of the journey. 



It seems to me that the contrast between this and Nageli's view 

 of the transmutation of species is obvious enough. According to him 

 the wanderer is not free to choose his path, but goes on and on along 

 a definite railway-line that only diverges here and there, and it 

 cannot be foreseen whether the track leads to paradisaic dwellings or 

 to barren wastes — the travellers must just make the best of what 

 they find. They carry a marvellous travelling outfit with them — 

 a sort of Tlschleln, deck' dich— the Lamarckian principle, but the 

 magic power of this is very doubtful, and it will hardly suffice to 



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