ALTERNATION OF GENERATIONS. 13 



leaves, from which emerge, not Neuroterus lenticularis, 

 but an insect hitherto considered as a distinct species, 

 belonging even to a different genus, Spathegaster bac- 

 carum. In Spathegaster both sexes occur ; they pro- 

 duce the currant-like galls found on oaks, and from 

 these galls Neuroterus is again developed. So also the 

 King Charles oak-apples produce a species known as 

 Teras terminal^, which descends to the ground, and 

 makes small galls on the roots of the oak. From these 

 emerge an insect known as Biorhiza aptera, which 

 again gives rise to the common oak-apple. 



Many butterflies, again, are dimorphic, existing 

 under two, or even three, distinct forms one that of 

 the winter, the other of the summer brood or broods. 

 Weismann has adduced strong reasons for thinking 

 that during the glacial period these species were one- 

 brooded only, and existed in the present winter form ; 

 that, as the climate improved, the period of warmth 

 became sufficient to allow the development of a second 

 brood, and led to the gradual rise of the summer form. 



He and Edwards have shown that, while, by the 

 application of cold, pupse, which would naturally have 

 produced the summer form, can be made to assume the 

 winter dress ; it is, on the contrary, far more difficult 

 to change the winter into the summer colouring. 



In some cases as for instance in the very curious 

 Leptodora crystallina (a fresh- water crustacean, in- 

 habiting deep lakes and reservoirs, and which, as its 

 name denotes, is almost perfectly transparent) though 

 the two forms are almost exactly similar in their mature 

 state, the mode of development is very different ; for, 

 while the winter form goes through a well-marked 



