176 DEVELOPMENT AND HEREDITY. 



like spots gave the caterpillar a reptilian aspect 

 calculated to terrify its feathered enemies. The 

 spots on the other segments, however, are useless 

 and inexplicable, I think, except in the way I have 

 suggested. In the same way among other arthro- 

 pods, where we find the appendages of one segment 

 developed in a peculiar manner for a particular pur- 

 pose, the appendages of adjacent segments tend to 

 show a similar modification of growth, though pur- 

 poseless and without particular function. For ex- 

 ample, the appendages of the crawfish behind the 

 pair of great claws are similarly modified in shape, 

 but are too small and weak to perform anything like 

 the same function. The large claws were undoubt- 

 edly developed by certain stimuli of growth, which 

 modified the co-ordinate forces controlling their 

 development ; but those co-ordinate forces con- 

 trolled not only the development of the large claws, 

 but also the development of the other appendages, 

 so that consequently the adjacent appendages became 

 secondarily modified in the same manner, but in a 

 lesser degree. This singleness of plan in living 

 matter, or the inability of one mass to contain more 

 than slight variations of one arrangement of devel- 

 opmental forces, is illustrated in a striking manner 

 in the similarity of the fore and hind limbs of 

 vertebrates, even when in man the uses of the two 

 pairs of limbs are so different. 



