Iiifrcdiicfion to Aiiinial Morplwlogy. 41 



being those whose variations of forms are correlated to 

 their external conditions giving them the advantage 

 over their congeners. The offspring of each individual 

 usually inherit the parental peculiarities often intensi- 

 fied. Those that do not are (by hypothesis) at a dis- 

 advantage, and die off; hence permanent varieties, 

 sub-species, and species arise. When we bear in mind 

 that the physical conditions of nature in any place have 

 been highly variable, it gives a greater force to this 

 explanation of the method of variation.* This is J/;''. 

 Darivin's theory of Natural Selection. As a natural 

 deduction from evolution, we have Dr. HangJiton' s 

 law, that " all structures are arranged so as to give the 

 maximum of work possible under the given external 

 conditions," a law which is at the basis of the curious 

 study of Dysteleology,t for by its operation it^ cuts 

 down all organs which, from aitered external condi- 



* But while explaining the method, Natural Selection does not explain 

 the cause. In artihcial selection the cause is the presidence and direction 

 of human intellect. In Natural Selection there is a necessity f5r predicating 

 the existence of a presidence similar in kind, but grander in degree, as the 

 changes effected by it are greater than those that artificial selection can 

 accomplish. This evolution theory explains the teleological correspondences 

 noticed in animals, for by hypothesis the organ progresses according as the 

 necessity for the function increases. Evolution also is morphological — that 

 is, structures progress only along certain lines, and of this we can give no 

 explanation, but can only say that it has been so — that is, evolution has 

 progressed under law. 



t Dysteleology is the study of rudimental and vanishing organs 

 {Haeckel). They must be distinguished from provisional organs, which are 

 structures useful in one period of life, disappearing in others. Rudiments 

 may be constantly or only occasionally present in the species. They may 

 be retrograded structures on account of parasitism, &c., or vanishing heir- 

 looms of foniier lower states. 



X I have here personified the law, as one must do in speaking of the 

 operations of law in general, a significant fact, as it testifies to our con- 

 sciousness that a law is nothing but the expression of a personal wiU, 



