History of the Theory of Heredity. 63 



fertilized female, and which gives rise early in the spring 

 to a brood of females without males. These at once lay 

 their eggs and form summer galls, from which both 

 sexes are born. 



In all cases the parthenogenetic forms are so different 

 from the sexual forms that they had previously been de- 

 scribed as distinct species, and in most cases they had 

 been placed in distinct genera. 



The following example selected from Adler's paper 

 'will give an idea of the character of his experiments: 

 Neuroterus lenticularis is a wasp which is born within 

 a small round gall which appears in July on the lower 

 surfaces of oak leaves. The galls continue to grow until 

 the end of September, when the leaves drop off and fall 

 to the ground. In the spring the insects escape, and all 

 of them are females, with their ovaries full of eggs, and 

 the male of this species was unknown previously to Ad- 

 ler's experiments. He gathered the fallen leaves, and 

 rearing the wasps in isolated captivity found that, soon 

 after the female is born, she pierces the leaf buds of 

 the oak, and lays her eggs. Adler marked by pieces of 

 thread all the buds which the insect was actually seen to 

 pierce, and in a few days he found on the leaves which 

 expanded from these buds a great number of minute 

 young galls, which soon became large enough to show 

 that they were very different from the winter gall in 

 which the parent was born. 



This new gall proved to be one with which entomolo- 

 gists had long been familiar, as the birthplace of what 

 had always been regarded as a wasp of quite a different 

 genus Spathogaster baccarum. It is a soft green gall, 

 punctated with red spots, and it grows entirely through 

 the leaf, so that part is on the upper and part on the 

 lower surface. The oak trees with these galls were kept 



