ct)0 H. H. Netvnian 



change. A character then may be conceived of as going through 

 a series of conditions before reaching the definitive state. It may 

 have started out as a dominant, become recessive for a time, 

 dominant again, and so on for a varying number of alternating 

 phases; and who knows whether the characters that we some- 

 times call definitive are really the end stages in the process ? 

 Possibly the further development of the individual as it reaches 

 maturity or senescence may show a condition of dominance less 

 dominant or even recessive, while some recessive characters may 

 come to light that before were unsuspected. If development is 

 continuous as long as life exists, and we have no reason to doubt 

 that such is the case, we should be somewhat cautious about our 

 statements with regard to the permanent dominance of this or 

 that character. If we limit our statements to some particular 

 period, such as early maturity or a larval condition, we would 

 avoid the danger of overstatement. 



The Importance of External Factors in Heredity 



Experience has taught us that only when we make every effort 

 to equalize the external conditions of a breeding experiment can 

 we expect to get anything approaching uniformity of develop- 

 ment. Slight differences in the physiological condition of the 

 parents, varying degrees of freshness of eggs or sperm, differences 

 in temperature and of water content, tend to produce differences 

 in the developmental process that cannot be attributed to the intro- 

 duction of foreign sperm. We are then driven to the conclusion 

 that uniformity of external conditions is as important a factor in 

 determining a similarity of offspring to parents as is uniformity 

 of germinal substance. 



Heredity is defined simply as a similarity of offspring to parents. 

 The question arises as to what constitutes this similarity. If the 

 germ cells from which the offspring develops are similar to those 

 from which the parent developed, and at the same time the external 

 conditions of development are similar, there results invariably a 

 similar developmental process, during which the offspring resem- 

 bles the parent stage for stage. This similarity of developmental 

 process, it seems to me, is the essence of heredity. The condition- 



