periodic variation of the latter:—certain determinants of one id would have adapted themselves to a high specific gravity; those in a second id to a lower specific gravity; and in a third, perhaps, to an intermediate condition. Along with this polymorphism a special sensitiveness of the primary constituents to varying degrees of saltness of the water would moreover have arisen, so that the right primary constituents would always be stimulated to development by the specific gravity itself.
Whether this is the right explanation, or whether and to what extent the direct influence of specific gravity co-operates through the action of intra-selection, cannot be deduced from the experiments that have so far been made; and I have only introduced this instance to point out that the way in which it is usually represented in order to illustrate the direct effects of external influences is by no means conclusive. It is quite conceivable that the saltness of the water in this case acts merely as a stimulus.
I cannot by any means claim to have exhausted my subject, and have only wished to give a few illustrations of how I imagine external influences to be made use of by nature for exciting the development of definite primary constituents of the germ. The subject is at present too new to permit of any enumeration of the influences on organisms which have been made use of as stimuli of double primary constituents; but we must a priori regard all kinds of influences as capable of being employed to regulate a potential development in certain circumstances. We see how
