016 APPENDIX B. 



be of no service unless the co-operative organs are changed. If, 

 for instance, there takes place such a modification of a rodent's 

 tail as that which, by successive increases, produces the trowel- 

 shaped tail of the beaver, no advantage will be derived unless 

 there also take place certain modifications in the bulks and shapes 

 of the adjacent vertebrae and their attached muscles, as well as, 

 probably, in the hind limbs ; enabling them to withstand the 

 reactions of the blows given by the tail. And the question is, 

 by what process these many parts, changed in different degrees, 

 are co-adapted to the new requirements whether variation and 

 natural selection alone can effect the readjustment. There are 

 threa conceivable ways in which the parts may simultaneously 

 change : (1) they may all increase or decrease together in like 

 degree ; (2) they may all simultaneously increase or decrease in- 

 dependently, so as not to maintain their previous proportions, or 

 assume any other special proportions ; (3) they may vary in such 

 ways and degrees as to make them jointly serviceable for the new 

 end. Let us consider closely these several conceivabilities. 



And first of all, what are we to understand by co-operative 

 parts ? In a general sense, all the organs of the body are co- 

 operative parts, and are respectively liable to be more or less 

 changed by change in any one. In a narrower sense, more 

 directly relevant to the argument, we may, if we choose to 

 multiply difficulties, take the entire framework of bones and 

 muscles as formed of co-operative parts ; for these are so related 

 that any considerable change in the actions of some entails 

 change in the actions of most others. It needs only to observe 

 how, when putting out an effort, there goes, along with a deep 

 breath, an expansion of the chest and a bracing up of the abdo- 

 men, to see that various muscles beyond those directly concerned 

 are strained along with them. Or, when suffering from lumbago, 

 an effort to lift a chair will cause an acute consciousness that not 

 the arms only are brought into action, but also the muscles of the 

 back. These cases show how the motor organs are so tied to- 

 gether that altered actions of some implicate others quite remote 

 from them. 



But without using the advantage which this interpretation of 

 the words would give, let us take, as co-operative organs, those 

 which are obviously such the organs of locomotion. What, 

 then, shall we say of the fore limbs and hind limbs of terrestrial 

 mammals, which co-operate closely and perpetually ? Do they 

 vary together ? If so, how have there been produced such con- 

 trasted structures as that of the kangaroo, with its large hind 

 limbs and small fore limbs, and that of the giraffe, in which the 

 hind limbs are small and the fore limbs large how does it 

 happen that, descending from the same primitive mammal, these 



