136 THE PROTOZOA 



is the banana-tree. In the wild-banana, seeds are produced from 

 flowers of a normal type by fertilization, just as in any other flower- 

 ing plant ; in the cultivated banana, however, the flowers are 

 sterile and incapable of fertilization, consequently the tree bears 

 fruit which are entirely seedless. Hence the cultivated banana- 

 tree is propagated entirely by a non-sexual method namely, by 

 the production of suckers growing up from the roots, and in no 

 other way. Whether this complete abolition of sexuality will in 

 time lead to exhaustion of the cultivated race of banana remains 

 to be seen, but at present there seem to be no signs of loss of vigour 

 under cultivation. 



If syngamy can be entirely dispensed with in an organism rela- 

 tively so high in the scale of life as a flowering plant, it seems 

 probable in the highest degree that the same may be true in many 

 cases for unicellular organisms of simple structure, and especially 

 for those parasitic forms which live, like cultivated plants, in a 

 medium rich in nutritive substances, and in an environment which 

 is changed at least once in each developmental cycle. Instances 

 of this are perhaps furnished by the various species of pathogenic 

 trypanosomes, strains of which have been brought to Europe and 

 propagated for many years from one infected animal to another 

 by artificial inoculation, without the natural agency of an inverte- 

 brate host. If it be true, as is generally believed, that in trypano- 

 somes syngamy takes place in the invertebrate host, then in the 

 long-continued artificial propagation of pathogenic trypanosomes 

 sexuality has been in abeyance for a vast number of generations 

 without any apparent loss of vital powers. The case of the patho- 

 genic trypanosomes cannot, however, be cited, in the present 

 state of our knowledge, as an absolutely conclusive example of 

 syngamy in abeyance, since it is by no means certain that this 

 process does not take place in the vertebrate host, where its 

 occurrence has frequently been affirmed (see p. 305, infra). But 

 it is certain that in trypanosomes generally, whether pathogenic 

 or non-pathogenic, syngamy is a rare phenomenon, since it has not 

 yet been demonstrated satisfactorily in a single instance, either in 

 the vertebrate or the invertebrate host, in all the many species 

 that have been studied. It is possible that, in these and many other 

 forms of life, sexual processes may intervene only at long intervals 

 in the life-history, and by no means in every complete cycle of 

 development or alternation of hosts. It then becomes necessary 

 to distinguish a developmental cycle, consisting of a recurrent 

 series of similar form-changes in regular succession, from a complete 

 life-cycle marked by the occurrence of an act of syngamy. In 

 such forms as the parasites of malaria, for example (p. 358), the 

 life-cycle and the developmental cycle coincide that is to say, 



