TRANSMISSIBILITY OF FUNCTIOXAL MODIFICATIONS 81 



developed in opposite directions. In the giraffe, for instance, the 

 fore-legs are longer than the hind-legs, which is the reverse of what 

 obtains in the majority of ruminants ; in the kangaroo the hind-legs, 

 on the contrary, have developed to a disproportionate size, while the 

 fore-legs have degenerated into relatively small grasping arms. 

 Co-operating parts, like the fore and hind limbs, may thus follow 

 opposite paths of evolution; their variations need not always be 

 directed to the same end. 



The difficulty presented by these so-called co-adaptations or har- 

 monious correlations cannot be denied, and we must also admit that, 

 if the results of exercise were inherited, the explanation of the phe- 

 nomenon would, in many cases— but not, indeed, in all— be easy, 

 because the adaptation of the secondarily varying parts in each 

 individual life would correspond exactly to the altered function of 

 the part, and would be transmitted to the descendants, and in them 

 would again be subject to such a degree of variation, according to the 

 principle of histonal selection, as might be conditioned by the further 

 progress of the primary variation. The simplicity of the explanation 

 is striking, if only it were at the same time correct ! But there are 

 whole series of facts, or rather of groups of facts, which prove that 

 the causes of co-adaptation do not lie in the inheritance of functional 

 modifications, and this must be recognized, even though we may not 

 yet be in a position to state the causes of co-adaptation, and to say 

 whether natural selection suffices to explain it or not. 



I must first point out that co-adaptations occur not only in 

 actively, hut also in passively functioning i^arts. Very numerous 

 instructive examples are to be found among the Arthropods, 

 whose whole skeleton belongs to this category. It has been 

 objected that this is not wholly passive, but that, like the bones of 

 vertebrates, it is stimulated by the contraction of the muscles and 

 incited to functional reaction, and that it thickens at places where 

 strong muscles are inserted, and becomes or remains thin where it is 

 not exposed to any strain from the muscles. But this is not the case, 

 for the chitinous skeleton can only offer resistance to the muscular 

 contractions when it is no longer soft, as it is immediately after it is 

 secreted. As soon as it has become hard, it can no longer be altered, 

 and can at most be worn away externally by long use. The proof 

 of this lies in the necessity for moulting, which is indispensable 

 to all Arthropods as long as they continue to grow, but does not 

 occur later. Every one who has followed the growth of an insect or 

 a crustacean knows well that the moultings or ecdyses are often 

 accompanied by great changes, and hardly ever occur without some 

 II. a 



