THE FUNCTIONS OF A LOCAL NATURAL HISTORY SOCIETY. 27 
on the roots of apple trees just beneath the surface of the ground, 
the animal and the Phylloxerze; these are also subject to an 
alternation of generations. Some of the gall mites also offer 
examples of this curious process, as fully formed eggs are to 
be found in the nymph, or that stage which answers to the pupa 
of a moth, 
The other term, ‘alternation of generations,” is applied to 
those cases in which the offspring is something quite different 
from the parent ; perhaps one of the best known and simplest 
cases of this is found in the history of the common tapeworn, to 
secure the full development of which two different species of 
animal hosts are necessary. The eggs of the mature worm have to 
be swallowed by a pig, from the stomach of which the embryoes, 
set free by the action of the gastric juice, bore their way into the 
muscular and other tissues, where they develop into small sac- 
like creatures known under the name of measles, and these undergo 
no further development as long as the host lives, but these 
measles, when swallowed in imperfectly cooked pork by a human 
being, are developed into the perfect tapeworm. Another instance 
is the destructive fluke ‘of the sheep’s liver, the life history of which 
is still more complicated, the creature undergoing part of its 
transformations free in the water, part in a small water snail, 
the Limnoea truncata, and the mature worm lives in the gall 
ducts of the sheep’s liver, and in this process the single egg would 
give rise to a multitude of individuals if all were to come to 
maturity. 
Now, in some of the species of the gall-making Hymenoptera, 
of the group of the Cynipideze, especially in one of the commonest, 
viz., that which produces the oak apple, only female insects are 
known, and this ignorance is not due to want of looking, for one 
observer collected 28,000 galls and bred from them 10,000 females 
without a single male. ‘To explain this fact, three theories are 
alone possible, either, firstly, that the insect is produced 
parthenogetically ; secondly, the male lives in galls on another 
species of plant, and differs in form and habits from the female ; 
or, thirdly, that it is a case of alternation of generations, the 
