38 General Part. 



ova again become asexual animals : or, two or more asexual 

 generations are followed by a sexual brood, these again by more 

 asexual generations. Sucb a regular alternation of sexual and 

 asexual broods is known as alternation of generations, 

 tbe generations may resemble one another, but usually they differ, 

 often very considerably (Hydroids, Tapeworms) . 



Heterogony. — The alternation just mentioned occurs between 

 sexual and asexual generations, but in many animals a generation 

 consisting exclusively of females, which reproduce parthenogenetically, 

 alternates with another which consists of males and females giving 

 rise to fertilised ova. There is generally some difference between the 

 virgin and the sexual generations. A simple form of such alternation 

 occurs in certain species of Gall- wasps, which cause oak-galls; a- 

 female generation here alternates regularly with a generation con- 

 sisting of males and females. The process is more complicated 

 in the Aphides, where, during the summer, several generations 

 of females appear in succession until, finally, a sexual generation 

 is produced ; the fertilised eggs rest through the winter and 

 become the first virgin brood of the next year; there are, 

 therefore, several parthenogenetic generations to each sexual genera- 

 tion. In the Vine-louse {Phylloxera) there is a further complication, 

 not only is the sexual very different from the parthenogenetic 

 generation, but of these one differs considerably from the rest. Such 

 an alternation of virgin and sexual (or hermaphrodite) generations 

 occurs not only among Insects but also in various Crustacea and 

 Platyhelminthia. 



Amongst those animals which reproduce only by fertilised ova, 

 successive generations are almost always alike ; a regular alternation 

 of generations of different appearance is found only exceptionally, 

 and in consequence of special conditions. In certain butterfiies, for 

 example, two generations, consisting of both males and females, occur 

 annually. The winter-brood, having wintered as pup^, appears in 

 the spring as perfect insects : their eggs give rise to the individuals 

 of the other generation, the summer-brood,* from which they differ 

 considerably in coloration (seasonal dimorphism). A hermaphrodite 

 Nematode {Rhahdonema nigrovenosum) , is parasitic in the lungs 

 of Progs and Toads j its embryos develop into another generation 

 which is free-living and of separate sexes, and which is essentially 

 different in appearance from the hermaphrodite generation. The 

 embryos of the free-living generation re-enter the Amphibia, and 

 become hermaphrodite worms like their grandparents. Some other 

 Nematodes exhibit the same peculiarity : an alternation of a 

 hermaphrodite generation, which leads a parasitic life, with a dioecious, 

 free-living generation. 



* Instead of one summer-brood, there may be two (i.e., three generations annually). 



