54 POPULAR SCIENCE MONTHLY. 



THE ORIGIN" OF SPECIES THROUGH SELECTION" CON- 

 TRASTED WITH THEIR ORIGIN THROUGH THE 

 APPEARANCE OF DEFINITE VARIATIONS. 



By Professor T. H. MORGAN, 



COLUMBIA UNIVERSITY. 



IT is a point of some interest that at the present time those zoologists 

 and botanists, who seem willing to transfer their allegiance from 

 Darwin's theory of natural selection to the theory of the survival of 

 mutations, often insist that the two points of view differ, after all, 

 only in degree and that selection is still the key note to the situation. 

 It seems to me, on the contrary, that there is a fundamental difference 

 between these two views, and in the hope of making this clearer I have 

 attempted in the following pages to contrast in certain respects the 

 applications that have been made of Darwin's theory with the im- 

 plications of the newer theory of the survival of definite variations. 



Attention has seldom, I believe, been called to the fact that only 

 those theories that have been advanced to account for the evolution 

 of animals and plants, have received wide recognition, that pretend to 

 explain how the adaptation of the organism to its surroundings has 

 been acquired. No such requirement is made in the case of theories 

 of evolution of the inorganic world. On what does this difference 

 depend? Why do we make certain demands in the case of organic 

 evolution that we do not make for the evolution of inorganic nature? 

 The answer in part is, that a living thing is unstable, it is easily de- 

 stroyed, and it must, if it is to maintain its integrity, be able to 

 respond to changes in the outer world in such a way as to keep the 

 balance that makes its existence possible. 



It is true that certain chemical substances are also highly unstable, 

 but we find in them no adjustments, no regulations, for maintaining 

 themselves, such as animals and plants exhibit. No theory, as I have 

 said, that pretends to account for the evolution of new organisms, has 

 been regarded as satisfactory unless it explained how the new forms 

 acquire those adjustments that make their life possible. How perfect 

 these adjustments must be is a question that has never been sufficiently 

 considered, partly because of the difficulties surrounding such an exam- 

 ination, and partly because of the widespread belief that living things 

 are as perfectly adapted to their environment as we can imagine 

 possible by the adjustment of their individual parts to the surroundings 



