GROWTH AND REPRODUCTION 43 



which it owes to its parents and their parents. It 

 is no exaggeration to say that if we could fully know 

 the inner nature of the processes that hold sway in 

 the dividing egg, we should hold the key to life as we 

 see it in the adult, for the macrocosm probably 

 has no powers that are not at least represented in 

 the microcosm. That enzymes have a large part in 

 determining the play of these developmental pro- 

 cesses is all but certain. The first stimulus to the 

 growth of the egg must be due to enzyme action. 

 It looks as if the question of sex were settled by the 

 enzymes with which a portion of the developmental 

 machinery is loaded. And finally it appears as 

 if the hereditary characters themselves were to be 

 referred to a definite mechanism controlled by 

 enzyme action. If we consider the mechanistic 

 hypothesis as it bears on the transmission of hered- 

 itary characters, we find it capable of bringing a 

 high degree of order and simplicity out of a tangle of 

 facts which the suppositions of vitalism left in a 

 state of confusion. 



One of the most helpful illustrations of what 

 biological science has gained from the mechanistic 

 hypothesis is seen in the Mendelian principle or law 

 in heredity. This law takes its name from a gifted 

 Austrian student and monk, Gregor Mendel, who 

 made some extremely acute and telling observations 

 on the transmission of hereditary characters in 

 plants. The principle which he found to hold good 

 for plants has been found to hold good in the hered- 



