ii2 THEORIES OF EVOLUTION 



an acquired character can be transmitted from the outlying 

 parts of the organism to its germ-cells, then we have 

 every reason for scrutinizing most carefully any evidence 

 that is alleged to prove such transmission. 



Let me first of all give you a concrete example which 

 is frequently brought forward by those in this country x 

 who believe in the Lamarckian Theory, and have chiefly 

 studied the skeletons of Mammalia. They say the joint 

 of an animal possesses just the sort of shape that would 

 be produced by the motion of the joint itself, and they 

 urge that the joint as we see it has arisen from the 

 hereditary effects of that motion. They look upon this 

 as a very satisfactory explanation, because they consider 

 it to be so obvious and fundamental. You do not require 

 anything further, selection is unnecessary, and even the 

 individual variation — so mysterious a factor of the 

 Darwinian Theory — is here entirely explained. 



But is the interpretation valid ? In the first place, it 

 is clear that such an hypothesis can never afford a wide 

 or general explanation. There are a great many parts 

 of the animal body which are not modified by their use. 

 You cannot thus explain the growth of hair, or the colour 

 upon the surface of the organism. For these and other 

 useful but passive structures the Lamarckian interpre- 

 tation will not hold at all. Hence we may divide the 

 organism into two sections, to one of which the Lamarckian 

 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, it is also necessarily 

 one which admits of convenient 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 those parts which the 



1 The United States. 



