INTRODUCTION. 



15 



the offspring generally resemble their parents. But 

 it is also a fact recognized by the stock-breeder, that 

 a particular animal wiU not only transmit several of 

 its own characteristics to its offspring, but perhaps 

 also various characteristics of the grandparents or of 

 animals belonging to stiU more remote generations, 

 although these characteristics are not visible in the 



Fig. Y.— The Small-winged Gall-fly, d (Andricus terminalis), lays its eggs separately 

 in the rootlets of oak. Eoot-galis (a) result from this, and inside of each of them a 

 larva develops which, after a metamorphosis, becomes a relatively large, wingless 

 gall-fly (c) known as BiorMza aptera. This pierces tlie oak-huds in early spring, 

 and lays a large number of eggs in them ; from part of the bud is formed a large 

 juicy gall (6), containing several larvse, from which the small-winged gall-flies (d) 

 develop. The species here represented exist, therefore, in two forms, e and d 

 (Heterogeny). 



animal which is actually breeding (Reversion, 

 Atavism). Among insects and the lower animals 

 there are species which, as adult animals, appear not 

 in one form, but two or several. In this case, as a 

 regular thing, the offspring does not resemble the 

 parents, but the grandparents, great-grandparents, or 



