12 INTRODUCTION. 



a single progenitor! So it is, if we look to the structure 

 of an individual animal or plant, when we see the fore and 

 hind limbs, the skull and vertebras, the jaws and legs of a 

 crab, the petals, stamens, and pistils of a flower, built on the 

 same type or pattern. During the many changes to which 

 in the course of time all organic beings have been subjected, 

 certain organs or parts have occasionally become at first of 

 little use and ultimately superfluous ; and the retention of such 

 parts in a rudimentary and utterly useless condition can, on 

 the descent-theory, be simply understood. On the principle of 

 modifications being inherited at the same age in the child, at 

 which each successive variation first appeared in the parent, we 

 shall see why rudimentary parts and organs are generally well 

 developed in the individual at a very early age. On the same 

 principle of inheritance at corresponding ages, and on the prin- 

 ciple of variations not generally supervening at a very early 

 period of embryonic growth (and both these principles can be 

 shown to be probable from direct evidence), that most wonderful 

 fact in the whole round of natural history, namely, the similarity 

 of members of the same great class in their embryonic con- 

 dition, — the embryo, for instance, of a mammal, bird, reptile, 

 and fish being barely distinguishable, — becomes simply in- 

 intelligible. 



It is the consideration and explanation of such facts as these 

 which has convinced me that the theory of descent with modi- 

 fication by means of natural selection is in the main true. 

 These facts have as yet received no explanation on the theory of 

 independent Creations ; 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 con- 

 tinued 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 mira- 

 culous creations having been necessary at innumerable periods ; 

 though this more simple view accords well with Maupertuis's 

 philosophical axiom " of least action." 



In considering how far the theory of natural selection may be 



