BIOLOGICAL SCIENCE 61 



knowledge gives into the problems of cross-breeding 

 has already resulted in the raising of new and valuable 

 types of wheat, and bids fair to revolutionise the 

 arts of the stock-breeder and horticulturist. In 

 mankind Mendelian relations have been established 

 for eye colour, where brown pigment in the iris is 

 dominant, and in certain diseases and malformations. 

 In these cases, the probable result of marriages of 

 affected persons can be estimated accurately, and the 

 proportion of their children likely to show the special 

 characteristic has become a matter of exact calcu- 

 lation. 



If our growing knowledge of heredity promises 

 greater final results, more practical gain has hitherto 

 been obtained from that branch of biology, which 

 has revealed to us the existence and life history of 

 those micro-organisms which have been proved so 

 powerful for good or ill to mankind. 



The Greeks and Romans had noticed and specu- 

 lated on some of the diseases which we now know 

 are caused by microbic action. The connection be- 

 tween marshy districts and malarial fever seems 

 always to have been recognised. Mildew or rust in 

 wheat had also attracted attention, and the Romans, 

 after the beginning of our era, associated its preva- 

 lence with inclement and damp weather, and with 

 ground that lay low and was exposed to heavy dews. 

 The diagnosis so far was correct, as well as the 



