It has been pointed out that in a number of species of animals and plants only parthenogenetic females are present at certain times. In a sense this means a preponderance of one sex, but since the eggs are adapted only to this kind of development, it may be claimed that the conditions in such cases are somewhat different from those in which eggs that would be normally fertilized may develop in the absence of fertilization. Nevertheless, it is generally supposed that the actual state of affairs is about the same. It is usually assumed, and no doubt with much probability, that these parthenogenetic forms have evolved from a group which originally had both male and female forms. One of the most striking facts in this connection is that in the groups to which these parthenogenetic species belong there are, as a rule, other species with occasional parthenogenesis, and in some of these the males are also fewer in number than the females. In the aphids, the parthenogenetic eggs give rise during the summer to parthenogenetic females, but in the autumn the parthenogenetic eggs give rise without fertilization both to males and to females. It appears, therefore, that we can form no general rule as to a relation between fertilization and the determination of sex. While in certain cases, as in the bees, there appears to be a direct connection between these two, in other cases, as in that of the aphids just mentioned, there is no such relation apparent. Geddes and Thompson have advocated a view in regard to sex which at best can only serve as a sort of analogy under which the two forms of sex may be considered, rather than as a legitimate explanation of the phenomenon of sex. They rest their view on the idea that living material is continually breaking down and building up. An animal in which there is an excess of the breaking-down process is a male, and one that is more constructive is a female. Furthermore, whichever process is in the excess during development determines the sex of the individual. Thus, if conditions are very favorable, there will be more females produced; but if, on the other hand, there is an excess of the breaking-down process, males are produced. So far, the process is conceived as a purely physiological one, but to this the authors then apply the selection hypothesis, which, they suppose, acts as a sort of break or regulation of the physiological processes, or in other words as a directive agent. They state: “Yet the sexual dimorphism, in the main, and in detail, has an adaptive significance, also securing the advantages of cross-fertilization and the like, and is, therefore, to some extent the result of the continual action of natural selection, though this may, of course, check variation in one form as well as favor it in another.” Disregarding this last addition, with which Geddes and Thompson think it necessary to burden their theory, let us return to the physiological side of the hypothesis.