THE THEORY OF HEREDITY 5 



The Theory of Heredity, lucidly expounded by Charles 

 Darwin, explains the perpetuation of like characteristics in 

 parents and offspring, " due to the capacity of every plant and 

 animal to produce other individuals of a like kind " {Spencer}. 

 "It is that science which deals with the organic relationships 

 of progenitors with descendants " (Reid). Variation is the 

 production of unlike characteristics. Hereditary similarity 

 is the usual rule, and variation the anomaly ; but " when a 

 new character arises, it generally tends to be inherited, at 

 least in a temporary and sometimes in a most persistent 

 manner." Such sudden changes are not due to the influence 

 of environment, but to " unknown laws acting on the organ- 

 isation or constitution of the individual " (Darwin}. Heredity 

 is evidenced in numerous ways, which require only to be 

 mentioned to be generally recognised. We may instance 

 the strong personal resemblance that runs through certain 

 families, not to mention races of men, e.g., the Jews, and through 

 breeds of horses,^., the Clydesdale and the Arab; the extra- 

 ordinary resemblance of the handwriting of three or four 

 brothers in one family, 1 seemingly the result of similar physical 

 structure of the hand and arm ; the tendency to the 

 production of doubles at birth, apart from the well-known 

 influence exerted by the abundance of food ; abnormal 

 growths of hair such as the grey patch on the neck and 

 shoulder of an Aberdeen-Angus heifer in Queensland 2 and 

 on a number of cattle in America, all descended from " Grey- 

 breasted Jack No. 2" (Vol. I. of the Herd Book}; the 

 tendency to specific disease like tuberculosis, and even 

 disease itself, appearing, like a splint or other bone affection, 

 and sometimes disappearing, like certain forms of chronic 

 indigestion in human beings, at a definite stage of the 

 development of the individual. As a set off to hereditary 

 defects, Darwin points out that "good health, vigour, and 

 longevity are equally inherited," and we may add, in cattle, 

 feeding and milking properties. Provisionally to explain why 

 like tends to produce like, Darwin propounded the theory of 

 Pangenesis. He assumed that gemmules, retaining the char- 

 acters of the original cells, are shed from every cell in the 



1 The Author's family is a case in point. 



2 Recorded in the Author's work on the Agriculture and Rural 

 Economy of Australia and New Zealand^ p. 413. 



