3° THE INVERTEBRATA 



experiments and observations have been made upon those creatures, 

 with a view to discovering the significance which the process has for 

 organisms in general. Most of these researches have been carried out 

 upon cihata. They have led to two theories: (i) that syngamy effects 

 a periodical rejuvenescence of the organism ; (2) that it produces new 

 types of individual and therefore gives the species more chances of 

 survival in the struggle for existence. 



(i) Cultures of protozoa in which conjugation is prevented are 

 liable after a time to fall into an unhealthy condition known as de- 

 pression, in which the nucleus^ is overgrown, the body stunted, divi- 

 sion retarded, and the various organs and functions increasingly de- 

 generate, until finally digestion ceases and the organisms die. From 

 this condition conjugation will recover a culture which is not too far 

 gone. It was held that depression was the senility of the organism — 

 ultimately of the same nature as that which in the Metazoa destroys 

 the parent body, while the gametes, after syngamy, continue the 

 existence of the species — and the conclusion was drawn that in both 

 cases the effect of the union of nuclei was rejuvenation. Now, how- 

 ever, it is known that depression is a disease, which by more natural 

 methods of culture can be avoided without conjugation. It is true that 

 in cultures of ciliates there has been observed a periodical waxing 

 and waning of vitality of which the low points in some cases coincide 

 with conjugation; but in other cases there occurs at these points not 

 conjugation but a process known as endomixis, which closely resembles 

 the procedure in conjugation, but takes place in solitary individuals 

 and does not involve syngamy. In this process (Fig. 28) the mega- 

 nucleus is destroyed and replaced by one of the products of the 

 division of the surviving micronucleus, as in conjugation. It would 

 appear from these facts that the invigorating effects of conjugation 

 are due not to the true syngamy (union of nuclei) but to the accom- 

 panying replacement of the meganucleus, which probably has become 

 effete (see p. 23). 



(2) Variety in a protozoan species is of three kinds : (a) that which 

 results from the production of different combinations of genes at 

 syngamy, and is permanent, forming races {pure lines) like those 

 which exist in higher organisms in the absence of cross -fertili- 

 zation; such pure lines have, for instance, been found in respect 

 of body-length in cultures of Paramecium, each line in the culture 

 breeding true so long as asexual reproduction continues; (b) that 

 which results from the spontaneous appearance of mutations ; this also 

 is permanent; it has been studied in Ceratium and other genera; 

 [c) that which results from modification of the individual by the direct 

 action of the environment; this, like mutation, produces differences 



^ In ciliophora the meganucleus. 



