STUDIES, SCIENTIFIC AND SOCIAL 



place by gradual steps through the ordinary processes of 

 generation is overwhelming. The numerous intermediate 

 links that have been discovered both among living and 

 extinct animals, and especially the wonderful community 

 perceptible in the embryological development of the most 

 diverse living types, force upon us the conclusion that the 

 entire animal and vegetable kingdoms owe the wonderfully 

 diversified forms they now exhibit to one unbroken process 

 of " descent with modification " from a few primeval types. 

 It is indeed generally assumed that if we go so far, we 

 must admit one original type of living organism ; but this 

 does not seem necessary. By means of whatever laws we 

 suppose living things first to have originated, why should 

 not the primeval germs have appeared several times 

 independently, and in forms determined or modified by 

 the infinitely varied chemical and physical conditions to 

 be found in the crust of the earth ? The identity of 

 ultimate structure and the wonderful similarities of de- 

 velopment of all organisms may be due to the unity of 

 the laws by which organic life was first produced; the 

 diversity of the great types of animal and vegetable forms 

 may be due to the operation of those laws at different 

 places, acting on different combinations of elements, which 

 are subject to unlike physical conditions. 



The point here insisted upon is, that the origin of all 

 organisms, living and extinct, by " descent with modifica- 

 tion," from one primeval germ or cell, is not necessarily 

 the same thing, and is not included in, " the origin of 

 species by means of natural selection." The latter we not 

 only know has occurred, but we can follow the process 

 step by step by means of known facts and known laws ; 

 the former, we are almost equally certain, has occurred, 

 but we cannot trace its steps, and there may have been 

 facts and laws involved of which we have no certain know- 

 ledge. The terms " laws of growth," " laws of develop- 

 ment," " laws of inheritance," " laws of variation," " laws 

 of correlation," " direct action of the environment," " laws 

 of habit and instinct," with some others, are used to 

 express the action of causes of which we are almost wholly 

 ignorant, as we are of the nature of life itself. Now Mr. 



