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SCIENCE. 



[N. S. Vol. XIV. No. 346. 



spoken of as adaptations. The most re- 

 markable fact in connection with these 

 adaptive responses is that they take place, 

 in some cases at least, in parts of the body 

 where they can never, or at most very 

 rarely, have taken place before, and the 

 regeneration is as perfect as when parts 

 liable to injury regenerate. Another im- 

 portant fact is that in some forms the re- 

 generation is so slow that if the competi- 

 tion amongst the animals was very keen, 

 those with missing legs, or eyes, or tails, 

 would certainly succumb ; yet if protected 

 they do not fail to regenerate. If, there- 

 fore, the animal can exist through the long 

 interval that must elapse before the lost 

 part regenerates, we can not assume that 

 the presence of the part is of vital impor- 

 tance to the animal and hence its power to 

 regenerate could scarcely be described as 

 the result of a ' battle for existence, ' and 

 without this principle ' natural selection ' 

 is powerless to bring about its supposed 

 result. 



It is extremely important to observe that 

 some cases, at least, of regeneration are 

 not adaptive. This is shown in the case 

 where a new head regenerates at the pos- 

 terior end of the old one in Planaria lugubris, 

 or where a tail develops at the anterior end 

 of a posterior piece of an earthworm, or 

 when an antenna develops in place of an 

 eye in several Crustacea. If we admit 

 that these results are due to some inner 

 laws of the organisms, and have nothing to 

 do with the relation of these organisms to 

 the surroundings, may we not apply the 

 same principle to other cases of regeneration 

 in which the result is useful ? 



So firm a hold has the Darwinian idea of 

 utilitarianism over the thoughts of those 

 who have been trained in this school, that 

 whenever it can be shown that a structure 

 or a function is useful to an animal it is 

 without further question set down as the 

 result of the death struggle for existence. 



A number of writers, being satisfied that 

 the process of regeneration is useful to the 

 animal, have forthwith supposed that, there- 

 fore, it must have been acquired by natural 

 selection. Weismann has been cited as an 

 example, but he is by no means alone in 

 maintaining this attitude. It would be en- 

 tirely out of place to enter here into a 

 discussion of the Darwinian theory, but it 

 be well worth while to consider it in con- 

 may nection with the problem of regener- 

 ation. 



We might consider the problem in each 

 species that we find capable of regenerating; 

 or if we find this too narrow a field for our 

 imagination we might consider the process 

 of regeneration to have been ' acquired by 

 selection in the lower and simpler forms ' 

 and trace its subsequent progress as it de- 

 creased in the course of phylogeny ' in cor- 

 respondence with the increase in complexity 

 of organization ' or with the decrease of ex- 

 posure to injury. At the risk of fol- 

 lowing the narrower point of view I shall 

 confine the discussion to the possibility of 

 regeneration being acquired, or even aug- 

 mented, through a process of natural selec- 

 tion in any particular species. 



The opportunity to regenerate can only 

 occur if a part is removed by accident or 

 otherwise. On the Darwinian theory we 

 must suppose that of all the individuals of 

 each generation that are injured hi exactly 

 the same part of the body, only those have sur- 

 vived or have left more offspring that have 

 regenerated. In order for selection to take 

 place it must be supposed that amongst 

 these individuals injured in exactly the same 

 region regeneration has been better in some 

 forms than in others, and that this difierence 

 is, or may be, decisive in the competition 

 of the forms with each other. The theory 

 does not inquire into the origin of this 

 difierence between individuals, but rests on 

 the assumption of individual differences in 

 the power to regenerate, and assumes that 



