22 THE POPULAR SCIENCE MONTHLY. 



crease of jaws and teeth in sundry kinds of pet dogs, has caused 

 great crowding of the teeth (The Factors of Organic Evolution, 

 pp. 12, 13). And I then argued that if co-operative parts, small 

 in number and so closely associated as these are, do not vary to- 

 gether, it is unwarrantable to allege that co-operative parts which 

 are very numerous and remote from one another vary together. 

 After making this rejoinder I enforced my argument by a further 

 example that of the giraffe. Tacitly recognizing the truth that 

 the unusual structure of this creature must have been, in its more 

 conspicuous traits, the result of survival of the fittest (since it is 

 absurd to suppose that efforts to reach a high branch could 

 lengthen the legs), I illustrated afresh the obstacles to co-adapta- 

 tion. Not dwelling on the objection that increase of any com- 

 ponents of the fore-quarters out of adjustment to the others would 

 cause evil rather than good, I went on to argue that the co-adapta- 

 tion of parts required to make the giraffe's structure useful, is 

 much greater than at first appears. This animal has a grotesque 

 gallop, necessitated by the great difference in length between the 

 fore and the hind limbs. I pointed out that the mode of action of 

 the hind limbs shows that the bones and muscles have all been 

 changed in their proportions and adjustments ; and I contended 

 that, difficult as it is to believe that all parts of the fore- quarters 

 have been co-adapted by the appropriate variations now of this 

 part, now of that, it becomes impossible to believe that all the 

 parts in the hind-quarters have been simultaneously co-adapted to 

 one another and to all the parts of the fore-quarters : adding that 

 want of co-adaptation, even in a single muscle, would cause fatal 

 results when high speed had to be maintained while escaping 

 from an enemy. 



Since this argument, repeated with this fresh illustration, was 

 published in 1886, 1 have met with nothing to be called a reply ; 

 and might, I think, if convictions usually followed proofs, leave 

 the matter as it stands. It is true that, in his Darwinism, Mr. 

 Wallace has adverted to my renewed objection and, as already 

 said, contended that changes such as those instanced can be 

 effected by natural selection, since such changes can be effected 

 by artificial selection : a contention which, as I have pointed out, 

 assumes a parallelism that does not exist. But now, instead of 

 pursuing the argument further along the same line, let me take a 

 somewhat different line. 



If there occurs some change in an organ, say, by increase of 

 its size, which adapts it better to the creature's needs, it is ad- 

 mitted that when, as commonly happens, the use of the organ 

 demands the co-operation of other organs, the change in it will 

 generally be of no service unless the co-operative organs are 

 changed. If, for instance, there takes place such a modification 



