understanding how such results are accomplished in nature or what other factors may be concerned.
It is certainly plain that the conditions surrounding sexual processes are immensely complex, and as yet we only know them in part and for a very few organisms. There is every reason to expect that investigation will so add to these that the subject will consist of very complicated problems in physics and chemistry. But it is something to know that important factors exist outside of the organism controlling in great part the sexual phase, and that some of them are so simple as light, temperature and osmotic pressure. Much is gained for biology in the understanding that sexual elements have arisen from asexual reproductive cells under the stress of environmental influences; that sexuality is not inherent in life although presented in almost all higher organisms, and that, however complicated the extreme conditions may be, they have arisen through a process of gradual evolution.
In another paper I shall hope to show the steps by which the highly differentiated egg and sperm in various groups of plants developed from the similar gametes presented at the dawning of sex. As stated in the beginning of this paper, the topic is a chapter in itself and well deserves separate treatment.