142 



FOREST ENTOMOLOGY. 



gall-fly — Neuroterus Icnticularis — deposited eggs in the bud and 

 produced the currant gall ; and the gall-flies of Spathegaster haccarum 

 or currant gall produced the spangle gall. In short, flies of the one 

 species deposited its eggs and produced the galls and flies of the other 

 species. Thus Adler demonstrated to the scientific world what had 

 only been previously guessed at — viz., the important biological law 

 of the alternation of generations in oak galls. This is a very important 

 biological law which is not uncommon amongst certain animals com- 

 paratively low in the scale of organisation, and is no doubt essential 

 to the wellbeing and continuation of species. In view, therefore, of 



Fig. \Z%— Galls 0/ Spathegaster baccarum i 

 rrocde flowers of oak. 



ISZ.— Galls 0/ Spathegaster 

 baccarum on haves. 



the biological importance of the subject, it may be well to contrast two 

 important and to some extent allied phenomena — viz.. Dimorphism 

 and Heteroecism — as compared with alternation of generations. 



Dimorphism means, two forms of the same thing. Thus certain 

 chemicals and minerals crystallise in two forms, and are called 

 Dimorphous, and if in three forms, Trimorphous. In animals we 

 have seasonable dimorphism, as change of colour in winter or 

 adaptation to environment ; sexual dimorphism, as instanced by the 

 difference between male and female in the peacock and pea-hen. In 

 the lower order of plants and animals many instances might be given. 



Heteroecism in animal and plant life means a change from host 

 to host in order to complete a life cycle. Thus the tapeworm found 



