12 INTRODUCTION. 



for classification, receive a rational explanation on the theory 

 of descent, combined with the principle of natural selection, 

 which entails divergence of character and the extinction of 

 intermediate forms. How inexplicable is the similar pattern 

 of the hand of a man, the foot of a dog, the wing of a bat, 

 the flipper of a seal, on the doctrine of independent acts of 

 creation ! how simply explained on the principle of the 

 natural selection of successive slight variations in the 

 diverging descendants from a single progenitor ! So it is 

 with certain parts or organs in the same individual animal 

 or plant, for instance, the jaws and legs of a crab, or the 

 petals, stamens, and pistils of a flower. During the many 

 changes to which in the course of time organic beings have 

 been subjected, certain organs or parts have occasionally 

 become at first of little use and ultimately superfluous ; and 

 the retention of snch parts in a rudimentary and useless 

 condition is intelligible on the theory of descent. It can be 

 shown that modifications of structure are generally inherited 

 by the offspring at the same age at which each successive 

 variation appeared in the parents ; it can further be shown 

 that variations do not coinnionbv supervene at a very early 

 period of embryonic growth, and on these two principles we 

 can understand that most wonderful fact in the whole circuit 

 of natural history, namely, the close similarity of the embryos 

 within the same great class — for instance, those of mammals, 

 birds, reptiles, and fish. 



It is the consideration and explanation of such facts as 

 these which has convinced me that the theory of descent 

 with modification by mean of natural selection is in the 

 main true. These facts have as yet received no explanation 

 on the theory of independent Creation; they cannot be 

 grouped together under one point of view, but each has to be 

 considered as an ultimate fact. As the first origin of life on 

 this earth, as well as the continued life of each individual, is 

 at present quite beyond the scope of science, I do not wish to 

 lay much stress on the greater simplicity of the view of a few 

 forms or of only one form having been originally created, 

 instead of innumerable miraculous creations having been 

 necessary at innumerable periods ; though this more simple 



