16 GENETICS AND EUGENICS 



genus throughout the long series of geological formations seems to have 

 been unbroken or continuous. New species come in gradually one by one. 

 Ancient and extinct forms of life are often intermediate in character, like 

 the words of a dead language with respect to its several offshoots or living 

 tongues. All these facts seemed to me to point to descent with modifica- 

 tion as the means of production of new species. 



The innumerable past and present inhabitants of the world are con- 

 nected together by the most singular and complex affinities, and can be 

 classed in groups under groups, in the same manner as varieties can be 

 classed under species and sub-varieties under varieties, but with much 

 higher grades of difference. These complex affinities and the rules for 

 classification, receive a rational explanation on the theory of descent, com- 

 bined 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 wliich m the course of time organic 

 beings have been subjected, certain organs or parts have occasionally be- 

 come at first of little use and ultimately superfluous; and the retention of 

 such 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 succes- 

 sive variation appeared m the parents; it can further be shown that varia- 

 tions do not commonly supervene at a very early period of embryonic 

 growth, and on these two principles we can imderstand that most wonder- 

 ful fact in the whole circuit of natural history, namely, the close similarity 

 of the embryos within the same 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 means of 

 natural selection is in the main true. These facts as yet received no ex- 

 planation 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 origm 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 origmally created, 

 instead of 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 ex- 

 tended; that is, in determining from how many progenitors the inhabitants 

 of the world have descended, — we may conclude that at least all the mem- 

 bers of the same class have descended from a single ancestor. A number 

 of organic beings are included in the same class, because they present, 



