Page:Popular Science Monthly Volume 34.djvu/831

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ON THE CAUSES OF VARIATION.

ment, passes away, the better for all concerned. It is one more of those wonders of which Byron wrote:

"Thus saith the preacher, 'Naught beneath the sun
Is new,' yet still from change to change we run;
What varied wonders tempt us as they pass!
The cow-pox, tractors, galvanism, and gas
In turns appear, to make the vulgar stare.
Till the swollen bubble bursts — and all is air!"

 — English Bards and Scotch Reviewers.


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ON THE CAUSES OF VARIATION.[1]

By C. V. RILEY, Ph.D., United States Entomologist.

II. — Concluded.

HAVING thus summarily indicated those factors of evolution associated with genesis and which are essentially physiological, however much psychical phenomena may co-operate, we may touch upon the more purely psychical factors or those pertaining to the growth and use of mind, employing the term to express those neural phenomena traceable to the medium of the brain. Their importance in evolution increases with increasing cephalization and complexity of nerve system. For the present purpose, however, it is with the objective side of psychology, or what may be called psycho-physiology, that we must deal.

Psychical — Use and Disuse. — Full consideration of the effect of use and disuse involves a discussion, not only of the question of the transmission of acquired structures, but of the influence of individual effort and of necessity — i. e., a consideration of the essentially Lamarckian factors in evolution. The occasion will not permit me to do full justice to these subjects. That functionally produced modifications are inherited as the great assumption upon which Lamarck founded his theory of evolution. Many able naturalists have insisted on it, and in my judgment there should no longer be any doubt whatever of the fact, not only so far as grosser structure is concerned, but brain-structure likewise. No question is of more moment in the whole range of biology, and especially biologic philosophy, and Spencer has well pointed out that on the answer to it will depend largely the sciences of psychology, ethics, and sociology. Weismann, Lankester, and others deny hereditary power in such modifications, the former believing that hereditary modification can result only from changes in the germ plasma, i. e., are virtually congenital. Natural selection, according to this view, plays upon the germ plasma; but I have

  1. From the address of the Vice-President of Section F of the American Association for the Advancement of Science, delivered at the Cleveland meeting, August, 1888.