THE GENESIS OF LIVING ORGANISMS 219 



of the genes. The genes themselves represent a union of a 

 latent ferment with an activating impulse. 



A gene has very different aspects according to whether 

 we regard it from the standpoint of plan or of causality. In 

 the former case, all we see are impulses which obey an extra- 

 material rule ; in the latter, we see a ferment which produces 

 certain physical and chemical effects in the protoplasmic foam. 



The genes are connected together by Mendel's law. In so 

 far as this relates to the mixing of the genes, it is a law of 

 probability, pure and simple ; in so far as it refers to the 

 dominance of certain genes, it is a biological law. Those 

 cases in which the dominance is not perfect and the recessive 

 character also finds expression (as in the Andalusian fowl, 

 where the blue feathers come from a crossing of white and 

 black parents) indicate that both genes are set in activity, 

 but, for the most part, the recessive ferment is blotted out by 

 the dominant. 



It helps us to understand this, if we imagine the genes to 

 be keys of a piano, only waiting to be struck for aU manner 

 of tunes to sound forth. We may consider the notes of the 

 score, which arrange for us the laws of a possible phenomenon ; 

 or we may listen to the playing, when the keys give out the 

 sound according to the law prescribed. 



The aim of descriptive biology must be to set down, by y 

 means of a kind of musical notation, the laws according to 

 which the genes in various animals sound together or in 

 succession ; this notation is imparted to us from the beginning 

 by the laws controlling the sounds emitted by the genes 

 in any animal, the genesis of which from the germ we are 

 attempting to observe. 



The description of the rule of genesis of some chosen 

 organism would represent just such a musical notation — but 

 nothing more. It would tell us nothing about the rule of 

 function in the fuU-grown subject, and nothing about the 



