RECENT ADVANCES IN ANIMAL BREEDING. 137 
hopeless confusion characterised the attempts that were made to homo- 
logise the body cavities in leeches so long as zoologists persisted in 
regarding the celom of that member of the Hirudinea with which they 
had been longest acquainted—the medicinal leech—as their starting- 
point, and in interpreting the state of affairs in other leeches in terms of 
this one. And the question remained in darkness until a few years ago, 
when Asajiro Oka showed that the coclom of the medicinal leech, far 
from being the starting-point, formed the very last term of a series of 
gradual modifications of the body cavity ; and in fact that the anatomy of 
this leech could not be understood without a knowledge of the series 
of which it was the culmination. 
References to recent literature on this subject will be found in a paper 
by me entitled “On the Difference between Physiological and Statistical 
Laws of Heredity ;’’ ‘‘ Manchester Memoirs,” vol. 1. (1906), No. 11. 
