PART III 



QUANTITATIVE METHOD AND PRIMORDIA 



§ 29.— SUMMARY OF PARTS I. AND II. {§§ 1-28).— The 

 living substance of each monotypic species (or subspecies) is a 

 mixture of a certain number of chemical entities (bioproteins ?), 

 each specific mixture differing from all others at least by one 

 component. When the qualitative composition of a specific 

 mixture is altered, a new specific form (sport, saltation or 

 mutation) is produced. 



Each observable property of a species is produced by a 

 reaction in which its living mixture as well as external causes 

 (conditions of existence) play a part. In each species the 

 number of possible reactions and therefore of possible pro- 

 perties is very large. In a given specimen a part only of the 

 possible properties is observable, the others being latent. 

 External causes (temperature, light, food, etc.), which are, of 

 course, very variable, determine which properties are observ- 

 able and which are latent in each specimen. This produces 

 variation (plasticity ; variants) within the limits of each species 

 without any qualitative alteration of its living mixture. 



External causes may produce a quantitative alteration of 

 a specific mixture. This alteration (and the corresponding 

 alterations of the observable properties) may be transmitted 

 by inheritance and augmented by (artificial) selection (but 

 never beyond the specific limits as long as the external causes 

 do not bring about a saltation). 



Another form of variation depends on complexity. A 

 species is complex when it consists of two or more subspecies 

 which differ from each other by quaUtative differences in the 

 constitution of their living mixture slighter than the specific 

 differences. Each subspecies is plastic just as is the species. 



In both plasticity and complexity the variation of a given 

 observable property may be continuous or discontinuous 

 (§ 21). 



In Part II. I have mentioned a number of facts which 

 prove that plasticity is greater than is actually realized by 

 most biologists. 



When we say that the (observable) properties of a species 

 are hereditary, we must add the proviso that they are trans- 

 mitted only when the external causes remain the same in the 



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