THE PRESIDENT'S ADDRESS. 79 



case with the specific characters of sponges, which, as we have 

 seen, depend for the most part upon trivial microscopical differ- 

 ences in the shape of the spicules. 



Without entering upon the vexed question of the relation 

 between somatogenic and blastogenic characters, we may assume 

 in our ignorance that such characters as those which we have 

 been discussing arise fortuitously in the germ-plasm, and that 

 it is a mere chance whether or not they may prove to be of any 

 value to the organism. If they are valuable, natural selection 

 will foster and encourage them ; if they are not, they may 

 nevertheless persist for many generations unless too injurious to 

 their possessors. If linked by correlation with useful characters 

 they may be indirectly fostered by natural selection, and un- 

 dergo a course of evolution parallel to that of their correlative 

 characters. Although they may be useless at first, they may 

 acquire some special value under new conditions of life, or in 

 the course of their evolution under the old conditions, and then 

 natural selection will begin to act upon them directly.* 



Possibly all the characters which an organism exhibits, with 

 the important exception of those which are due to the effects 

 of use and disuse of organs, or to the response of the 

 organism in some other way to the direct action of the environ- 

 ment, have first arisen as by-products of the complex chemical 

 and physical processes upon which the life of the organism 

 depends. 



There is one more aspect of the problem to which I should like 



* Having been asked to give a definite example of a character which, 

 at first useless, has ultimately acquired an adaptive value, I suggest the 

 pattern of the venation on the front wings, or tegmina, and on the leaf-like 

 outgrowths of the abdomen in the leaf-insect Pulchriphyllium cvurifoliuvi. 

 This venation so closely resembles that of a leaf as greatly to increase 

 the remarkable protective resemblance which undoubtedly enables the 

 insect to conceal itself effectively from its enemies. The mere pattern 

 of the venation in the more primitive and typical Orthoptera can hardly 

 have had any selective value. Of course the venation itself must always 

 have been useful, both for supporting the wings and for supplying them with 

 air, etc. ; but as regards the pattern which the venation makes (which is 

 the character to which I refer) one type of arrangement would seem to 

 have been as good as another until it acquired a special adaptive value 

 as a factor in bringing about protective resemblance to a leaf, and then 

 doubtless the pattern evolved under the influence of natural selection until 

 it reached its present degree of perfection. 



