524 LIFE : OUTLINES OF GENERAL BIOLOGY 



as well. Cu^not found that the proportion of the sexes in blowflies, 

 where its visible determination is later than in Lepidoptera, was not 

 affected by what the larvae ate, or by what their parents ate. 



What then is our conclusion in regard to the first theory ? Perhaps 

 there is no quite convincing evidence to show that environmental 

 influences operating on a developing organism may decide what its 

 sex is to be. Yet we should be slow to assert that this is impossible. 

 Consider, for instance, Nussbaum's elaborate experiments on Hydra 

 grisea, which he subjected to varying nutritive conditions. In this 

 species there are both hermaphrodite and ditecious forms. Nussbaum 

 found that the optimum nutritive conditions resulted in a pre- 

 dominance of female polyps, and that groups wholly male could be 

 produced by relative starving. 



According to Baltzer, but pace Goldschmidt, the free-swimming 

 larva of the Bonellia worm that settle down on the proboscis of 

 old females develop into pigmy males ; those that sink into the sub- 

 stratum develop into large females. 



There are analogous experiments in regard to some plants. Thus 

 Prantl found that spores of the Royal Fern (Osmunda) and of 

 Ceratopteris thalictroides, sown in soil without nitrogenous supplies, 

 developed into male prothalli, that female organs were formed 

 when ammonium nitrate was supplied, and that wholly male 

 prothalli might become wholly female prothalli. In cases like fern- 

 prothaj^ and Hydra, which may be normally hermaphrodite, what 

 actually occurred in the experiments was probably the inhibition 

 or suppression of one set of sexual organs in favour of another. 

 Nevertheless the experiments suggest that the first theory (a) is 

 not to be dismissed too hurriedly. 



Moreover, when we recall how a little nutritive attention makes a 

 worker-grub a queen-l>ee, or how Aphides produce females partheno- 

 genctically through months (or even years) of high feeding and 

 pleasant temjxTature, and how the advent of autumn, with its cold 

 and its scarcity of food, is followed by a birth of males, and so on, 

 we may not be able to share the dogmatism of some who assert 

 that the theory of the environmental determination of sex is pre- 

 posterous. We shall afterwards consider the question of the influence 

 of the environment on the parents. 



[b] SECOND THEORY— 77/fl/ the sex is utidetermined until the 

 germ-cells unite in fertilisation, when it is decided by their relative 

 condition, or by a balancing of the tendencies they bear, neither sperm 

 nor ovum being necessarily decisive. 



It has been a favourite theory, especially in regard to man and 

 mammals, that the sex of the offspring depends upon the relative 

 condition of the germ-cells at fertiUsation, the differences in 

 condition depending on the relative age of the parents and on 



