270 LECTUEES ON BIOLOGY 



which fly from vine to vine, laying eggs here and there, partly 

 large, partly small. 



This generation, too, reproduces parthenogenetically, but 

 from their eggs the sex-animals are developed, from the large 

 eggs, the females; from the small, the males. They are most 

 remarkable creatures, and can only be properly described as 

 walking sex-organs. They have not only no wings, but are also 

 without proboscis and intestinal canal. They are 'ephemeral,' 

 unable to feed themselves, and their only object appears to be 

 to alternate the succession of asexual generations by fertilization. 

 Each female deposits under the bark only one egg and with that 

 its life-task and its life is ended. These are the ' winter eggs ' 

 from which the first rays of the spring sun call forth the young 

 parthenogenetic generation of females. 



It is only in a few cases where it may be so clearly observed 

 that fertilization and reproduction, though closely related, are 

 yet processes of a different kind. In Phylloxera fertiliza- 

 tion does not lead to multiplication, for as each female produces 

 only one fertilizable egg, the appearance of a sex-generation 

 reduces the number of individuals in fact by half. The im- 

 portance of fertilization does not consist in reproduction, as 

 was formerly believed, but in bringing about amphimixis a 

 mixing of the qualities. Development can, therefore, take 

 place without previous fertilization, and, on the other hand, 

 fertilization need not necessarily cause development or repro- 

 duction. But fertilization is of the utmost importance to organ- 

 isms, as it imparts to them a far greater adaptability, and, on 

 the other hand, suppresses, or at any rate hinders, an injurious 

 one-sided tendency to variation. Arrangements have therefore 

 subsequently arisen which as a rule prevent the independent 

 development of the germ-cells. Thus the inability of sperm and 

 ovum to proceed separately to development is an adaptation 

 to the demands of life. 



Heterogony proceeds largely in a like manner in the parasitic 

 Trematodes of which we have heard before. As may be 

 remembered, in the larval forms of the Liver Fluke sporocysts 

 and rediae produce asexually numerous larvae which are all able 

 to develop into sexual mature animals. This process is not to be 

 conceived as interior gemmation, as was formerly believed, but 



