PASSIVE AND ACTIVE STRUCTURES 113 



Lamarckians also claim to explain, while it offers the 

 only explanation of the other parts. 



If we suppose that Lamarckian Evolution in part 

 explains the actively used organs, and Darwinian Evolu- 

 tion in part, we should expect that modification would 

 take place more quickly in that section of the organism 

 where the two principles were at work than in the other 

 section where only one principle — the Darwinian — can 

 play a part. But there is no evidence of such especially 

 rapid evolution. It seems to me that we are in a position 

 to use the old principle of cutting off superfluous causes. 

 No unnecessary cause should ever be introduced into an 

 explanation, and if Lamarckism, untenable in the one 

 section, is superfluous in the other, it should be removed, 

 unless there is very clear evidence proving that it has 

 been at work. 



Furthermore, in certain cases, such as the protective 

 attitudes and appearances assumed by many animals, we 

 meet with clear evidence that the two kinds of parts — 

 those that are affected by their use and those that are 

 not affected — have undergone development together, 

 suggesting strongly that their evolution has been under 

 the direction of one set of forces, and not of two sets 

 which have little in common. 



Having now brought forward certain general objections 

 to the Lamarckian position, let me take exception to one 

 or two special cases. 



Certain animals, such as lobsters and crabs, have the 

 power of very readily parting with some of the most 

 important of their members. The large claws are easily 

 thrown off, and this may be of great advantage, in the 

 struggle for life, because when an individual is attacked, 

 and has seized the enemy with its claw, it has a chance of 

 escaping. In the case of the lobster, the dismembered 

 claw may not let go of the enemy although the enemy 

 may wish to let go of the claw. The claw may take 

 charge of the enemy while the lobster escapes. 



Now that is a very interesting adaptation. We find 

 the claw so constituted that it can be thrown off, but even 

 when thrown off it continues to be of much use to the 



