6 INTRODUCTION TO SEXUAL PHYSIOLOGY 



Paramoecium senile degeneration could be avoided by supplying 

 the culture with beef extract, and various observers have found 

 that some small alteration in the environment, such as the addition 

 of a minute quantity of alcohol or even a slight change in the 

 composition of the water, may prevent deterioration from setting 

 in and so admit of the continuation of binary fission without 

 resorting to conjugation. As we shall see later, there is some 

 evidence that whereas continuous and close inbreeding among 

 the higher animals may lead to general deterioration and sterility, 

 these consequences may be obviated and a relatively infertile 

 race rejuvenated by access to a new environment. 



On the other hand, conjugation among the Protozoa does not 

 seem to be necessarily associated with reproduction, and accord- 

 ing to Enriques, conjugation only takes place with Colpoda 

 steini under certain peculiar environmental conditions {e.g. if 

 the layer of the water is less than 2 millimetres in thickness), 

 and it has been suggested that the utility of the process depends 

 upon the power given thereby to withstand adverse circumstances. 

 It would seem also that the only feature common to conjugation 

 and fertilisation among the higher animals is biparental inheri- 

 tance which usually im})lies variation. Amid adverse conditions 

 of existence conjugation is often more apt to occur, and it is 

 possible that among the new combinations of variations brought 

 about as a result of the process, there are some which are better 

 suited to the altered circumstances, and that the advantage 

 gained in this w^ay is an important factor in the survival of the 

 race. (See below, p. 123.) 



Sexual Reproduction among Multicellular Animals. — With the 

 species of Protozoa above cited the two conjugating cells unite, 

 and each may be said to fertilise the other through the exchange 

 of nuclear or other protoplasmic material or as a result of per- 

 manent union which is preceded by binary fission. In other 

 species, however, such as Vorticella or the common bell-animalcule, 

 there are two sexual individuals, one of which is attached by a 

 contractile stalk to a water weed, and another smaller individual 

 which is free-swimming and bores its way into the larger stationary 

 onein the act of conjugation. Herewe have adistinct foreshadow^- 

 ing of the differentiation of sex cells, which is characteristic of 

 all the Metazoa or multicellular animals. We find also dimorphic 

 reproductive cells in Volvox which consists of a colony of cells 



