In unicellular plants, and in some protozoans, two individuals physically alike, or nearly alike, and moving about in water, unite to form a single individual. In animals more complicated entire individuals no longer merge into one; but this function has become restricted to certain cells, just as the function of moving the animal from place to place has become restricted to certain cells. In the majority of animals living in water these cells are liberated in the water, and here two cells unite, as in lower forms two individuals unite.
The cells uniting are equally important in transmitting hereditary characters, and in their essential structure. As an adaptation to the necessity of the union of two cells and to the necessity that the combined volume of the two cells be sufficient to give the new individual a fair start, the cells, as the result of a division of labor, have become very different in shape and action. One has taken upon itself the function of providing the nutriment necessary to start the new individual,
 | ||
| Fig. 7. | Fig. 8. | |
Fig. 7. Two Typical Cells; Ovarian Eggs of Cymatogaster.
Fig. 8. Photograph of the Conjugation of Two Strings of Individuals of Spirogyra; at a the contents of one individual are passing over into the next; at b the contents of two individuals are united within the wall of one; at c the two uniting individuals have been metamorphosed into a spore; d, a bachelor which missed a mate.
and has become comparatively large and inactive. The other has taken upon itself the function of providing the mobility necessary to insure the union of the two cells. It has become excessively minute, and very mobile. The differences in the cells concerned in sexual reproduction extend to the ducts through which the cells are emitted, and secondarily to the whole individual, so that in every organ, function and psychical trait male and female are different.
Into the question of the advantages of the production of a new individual by the union of two cells we can not enter in detail. Suffice it to say that on the part of some it is looked upon as the union of two hereditary tendencies, which eliminates extreme badness, or what amounts to the same thing, extreme goodness that may be inherent in one of these tendencies, and at the same time insures new combinations of characters from which nature may select the fit.[1] On the
- ↑ Pearson has demonstrated mathematically by comparing parents with offspring that 'whatever be the physiological function of sex in evolution, it is not the production of greater variability.'
