10 HEREDITY 



Darbishire has performed the experiment of crossing 

 some ordinary white mice with others of the Japanese 

 waltzing kind. The peculiarity of the latter is their 

 habit of dancing or waltzing i.e. continually turning 

 round in one direction until they are exhausted. The 

 cross-bred mice showed no inclination whatever to 

 waltz it appeared as though the habit were lost alto- 

 gether. But these cross-bred mice were mated together 

 and the next generation was reared. A large propor- 

 tion again showed no tendency to " waltz," but a certain 

 number were found to have the habit just as strongly 

 developed as their original waltzing grand-parents. On 

 counting it was found that there were about three 

 ordinary mice to each one which " waltzed." 



In the course of a number of experiments in the 

 crossing of sweet-peas, Bateson and Punnett crossed 

 two individuals of the " Emily Henderson " variety. 

 This variety has always pure white flowers, and the 

 only difference between the two plants was in the 

 shape of the pollen-grains, one having round grains, 

 and the other oval. The cross-bred plants from the 

 two white-flowered parents had deep purple flowers. 

 The seed from these plants was again sown, and in the 

 next generation both white-flowered and purple-flowered 

 plants were found. On counting it was found that there 

 were about seven of the former to nine of the latter. 



One might greatly multiply such examples, but those 

 given will serve to show how remarkable, and how 

 apparently contradictory, the facts of heredity are. 

 It is the purpose of the science of heredity to explain 

 all these facts according to one consistent theory. 



The science of heredity is at once one of the oldest 

 and one of the youngest of the sciences. The old Greek 

 philosopher and doctor Hippocrates propounded a 

 theory of heredity on something of the same lines as 

 Darwin's, yet most of the writings before the time of 

 Darwin, and a good deal of what has been written 

 since, bear about the same relation to the science of 

 heredity as do the writings of the alchemists to modern 

 chemistry. 



