Page:Popular Science Monthly Volume 67.djvu/60

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54
POPULAR SCIENCE MONTHLY.

THE ORIGIN OF SPECIES THROUGH SELECTION CONTRASTED 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 implications 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 destroyed, 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 examination, 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