ENVIRONMENTAL FACTORS 491 



true of lower organisms such as Rotifers, Entomostraca, worms 

 and polyps, and some plants. 



Issakowitch has recently maintained in regard to the common 

 " water flea," Cyclops, and Von Malsen in regard to one of the 

 simple worms, Dinophilus apatris, that abundance of food and a 

 low temperature result in the production of a large number of 

 females. This may be due to preponderant production of a 

 particular kind of egg, — that tending to develop into a female. 

 Nussbaum maintains in regard to the Rotifer, Hydatina senta, 

 that the nutrition of the female embryo determines whether it 

 will produce large eggs — developing into females, or small eggs 

 developing into both males and females. 



We may also refer to the experiments of Klebs [Die 

 Bedingimgen der Fortpflanzung hei einigen Algen und Pilzen, 

 Jena, i8g6), where it is shown that definite environmental con- 

 ditions of nutrition, temperature, etc., are definitely associated 

 with the occurrence of particular modes of reproduction in Algae 

 and Fungi. A Vaucheria plant, kept sterile for years, can be 

 made to become sexual in a few days. A form normally bisexual 

 can be made unisexual. Asexual spore-formation can be induced 

 with certainty by one set of conditions {e.g. in Hydrodictyon) , and 

 the appearance of sexual gametes by another set of conditions. 

 Only by definite experiments like those of Klebs can we pass 

 from vague interpretations to a precise physiology. 



§ 4. I'dflnence of the Parents 



There seems no satisfactory evidence, as yet, that the sex of 

 the offspring can be influenced by environmental conditions, such 

 as those of nutrition and temperature, operative on the develop- 

 ing organism either directly or indirectly. It may be, however, 

 that the bodily constitution of the parent may have an influence 

 on the germ-cells, giving them a bias towards the production of 

 male or female offspring. Does an aged parent tend to have 



