150 



SCIENCE: 



[N. S. Vol. XX. No. 500. 



them evidence in favor of selection, as well as 

 by those to whom Darwinian explanations 

 seem absurd. In this latter group is Professor 

 T. H. Morgan, whose books, ' Eegeneration ' 

 and ' Evolution ' and Adaptation,' * assert 

 the inadequacy of selection. As the work 

 summarized in the first volume has inspired 

 the point of view from which the second one 

 was written, a careful criticism of the former 

 is a test of the soundness of the latter. Such 

 criticism is difficult, not only from the nature 

 of the subject, but especially because of a 

 paradoxical frame of mind due to my agree- 

 ment with Professor Morgan's main conten- 

 tion without being able to accept his own 

 reasons for it. 



Autotomy. — ^Professor Morgan regards the 

 process of autotomy as a fatal stumbling 

 block for the theory of natural selection. 

 Thus on page 155 of 'Regeneration' we read: 



Even if it were granted that the theory of 

 natural selection is correct, it does not follow 

 that all useful processes have arisen under its 

 guidance. We may, therefore, leave the general 

 question aside, and inquire whether the process 

 of autotomy could have arisen through natural 

 selection (admitting that there is such a process 

 for the sake of the present argument), or whether 

 autotomy must be due to something else. 



If we assume that the leg of some individual 

 Cray fishes or crabs, for example, broke ofif, when 

 injured, more easily at one. place than at another, 

 and that regeneration took place as well, or even 

 better, from this region than from any other, 

 and if we further assume that those animals in 

 which this happened would have had a better 

 chance of survival than their fellows, then it 

 might seem to follow that in time there would 

 be more of this kind of animal that survived. 

 But even these assumptions are not enough, for 

 we must also assume that this particular varia- 

 tion was more likely to occur in the descendants 

 of those that had it best developed, and that 

 amongst those forms that survived, some had the 

 same mechanism developed in a still higher degree, 

 and, the process of selection again taking place, 

 a further advance would be made in the direction 

 of autotomy. This, I think, is a fair, although 

 brief, statement of the conventional argument as 



* ' Regeneration,' by Thomas Hunt Morgan, The 

 Macmillan Company, New York, 1901. ' Evolu- 

 tion and Adaptation,' by Thomas Hunt Morgan, 

 The Macmillan Company, New York, 1903. 



to how the process of natural selection takes place. 

 But let us look further and see if the results 

 cbuld be really carried out in the way imagined, 

 shutting our eyes for the moment to the number 

 of suppositions that it is necessary to make in 

 order that the change may occur. It will not 

 be difficult, I believe, to show that even on these 

 assumptions the result could not be reached. In 

 the first place, the crabs that are not injured in 

 each generation are left out of account, and 

 amongst these there will be some, it is true, that 

 have the particular variation as well developed 

 as the best amongst those that were injured, and 

 others that have the average condition, but there 

 will be still others that have the possibilities less 

 highly developed, and the two latter classes will 

 be, on the hypothesis, more numerous than those 

 in the first class. The uninjured crabs will also 

 have an advantage, so far as breeding and resist- 

 ing the attacks of their enemies are concerned, 

 as compared with those that have been injured, 

 and in consequence they, rather than the injured 

 one, will be more likely to leave descendants. 

 Even if some of those that have been injured, 

 and have thrown off the leg at the most advan- 

 tageous place, should interbreed with the unin- 

 jured crabs, still nothing, or very little, can be 

 gained, because, on Darwinian principles, inter- 

 crossing of this sort will soon bring back the ex- 

 treme variations to the average. 



The process of natural selection could at best 

 only bring about the result provided all crabs in 

 each generation lose one or more of their legs, 

 and amongst these only the ones survive that 

 break off the leg at the most advantageous place; 

 but no such wholesale injury takes place, aa 

 direct observation has shown. At any one time 

 only a small percentage, about ten per cent., have 

 regenerating legs, and as the time required com- 

 pletely to regenerate a leg, even in the summer, 

 is quite long, this percentage must give an ap- 

 proximate idea of the extent of exposure to in- 

 jury. It is strange that those who assert off- 

 hand that, because autotomy is a useful process, 

 therefore it must have been acquired by natural 

 selection, haVe not taken the pains to work out 

 how this could have come about. Had they done 

 so, I can not but believe they would have seen how 

 great the difficulties are that stand in the way. 



A further difficulty is met when we find that 

 each leg of the crab has the same mechanism. 

 If we reject as preposterous the idea that natural 

 selection has developed in each leg the same struc- 

 ture, then we must suppose that a crab varies in 

 the same direction in all its legs at the same time; 

 and if this is true it is obvious that the prin- 



