specific Characters Composite 13 



factors is necessary that we almost shrink from the con- 

 sequences of an analysis, it is clear, on the other hand, 

 that, for the building up of the sum total of all organisms, 

 there is required a rather small number of individual 

 hereditary characters in proportion to the number of 

 species. Regarded in this way, each species appears to us 

 as a very complex picture, whereas the whole organic 

 world is the result of innumerable different combinations 

 and permutations of relatively few factors. 



These factors are the units which the science of hered- 

 ity has to investigate. Just as physics and chemistry go 

 back to molecules and atoms, the biological sciences have 

 to penetrate to these units in order to explain, by means 

 of their combinations, the phenomena of the living world. 



Phylogenetic considerations lead to the same conclu- 

 sions. Species have gradually been evolved from simpler 

 forms, and this has taken place by the addition of more 

 and more new characteristics to those already existing. 

 The factors which compose the character of a single spe- 

 cies are, therefore, in this sense, of unequal age ; the char- 

 acteristics of the larger groups being in general, older 

 than those of the smaller systematic divisions. But the 

 very consideration that the characteristics have been ac- 

 quired singly or in small groups, shows us again from 

 another side their mutual independence. 



It is a striking, yet by far insufficiently appreciated 

 fact that frequently, in distant parts of the genealogical 

 tree, the same character has been developed by wholly 

 difiFerent species. Such "parallel adaptations" are ex- 

 tremely numerous, and almost every comparative treat- 

 ment of a biological peculiarity shows us examples 

 thereof. The insect-eating plants belong to the most 

 varied natural families, vet thev all possess the power of 



