Page:Popular Science Monthly Volume 15.djvu/162

150 can be made without difficulty, and will be found to show that variation is much more marked and common in plants raised from fertilized seed than in those raised by budding. A marked bud-variation is of very rare occurrence, but in many cases the tendency of plants raised from seeds to differ from the parents is so great that choice varieties are propagated entirely by buds. It is almost hopeless to attempt to propagate a choice variety of grape or strawberry by seeds, as the individuals raised in this way seldom have the valuable qualities of their parents, and, although they may have new qualities of equal or greater value, the chances are of course greatly against this, since the possibility of undesirable variation is much greater than the chance of a desirable sport. There is no difficulty, however, in perpetuating valuable varieties of these plants by asexual reproduction.

Putting together these various propositions—that the evolution of life has been brought about through the combined action of the law of heredity and the law of variation; that in all except the simplest organisms the process of sexual reproduction by ova which have been acted upon by the male element is met with; that the ovum is alive, and capable of development in itself, and that the essential function of the male element is something else than the vitalization of the ovum; that the process of sexual reproduction differs from the process of asexual reproduction only in the occurrence of impregnation, while the result of the former process differs from the result of the latter in its greater variability—we seem warranted in concluding that the ovum is the material medium through which the law of heredity manifests itself, while the male element is the vehicle by which new variations are added. The ovum is the conservative, and the male element the progressive or variable factor in the process of evolution of the race as well as in the reproduction of the individual. The adequate statement of the evidence upon which this generalization rests, or even a full statement of the generalization itself, with its qualifications, would be out of place here, but the facts which have been given seem to be sufficient to warrant its use as one step in our argument in regard to the relations of the sexes. From this as our basis we will now trace the evolution of sex.

Among the lowest organisms, animal and vegetable, multiplication is usually by the various forms of asexual generation, budding or fission, or cell-multiplication—an organism which has by ordinary growth increased in size beyond the limit of exact harmony with its environment, dividing in this way into two, like each other as well as like their parent. In this way the preservation of the established characteristics of the species—heredity—is provided for, but in order that progress should take place, by the preservation of favorable varieties, variation must also be provided for. This is accomplished by the process which is known as conjugation: two protoplasmic organisms approach, come into contact, and a transfusion or mixture of the