1894.| 387 [| Poulton, 
theory might be held to apply, and to the other the Darwinian 
alone. 
But upholders of the Darwinian theory consider that it applies 
to the other section as well. They point out, that while the 
form of the joint is the sort of form that would be produced by 
the motion, such a form is the only one which admits of con- 
venient motion, that motion has been essential to the life of the 
organism, that alert and rapid movements have been a necessity 
in the struggle for existence, and that any form which would 
prevent or clog the movements would be at once destroyed by 
the operation of natural selection. Natural selection they hold 
to be competent to explain these parts which the 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 evolution 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 Dar- 
winian—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 explana- 
tion, and if Lamarckism, untenable in the one section, is super- 
fluous 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 effected by 
their use and those that are not affected—have undergone develop- 
ment 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 of life, because when an 
