32 THE ORGANIC GROWTH OF THE LIVING WORLD sec. 



generations it appears always a little earlier, until it itself 

 characterises the greatest part of the period of growth. This 

 " law of precocious inheritance " as Wlirtenberger calls it, also 

 holds — for it obviously coincides with the law of abbreviated 

 development — for living animals.^ 



" The agreement of the palseontological facts afforded us 

 by the Ammonites with those supplied by living animals," 

 I have said,- certainly with justice, " is in the highest degree 

 interesting. It places the perfectly general application of the 

 law beyond all doubt." ^ For not only the markings, but all 

 other characters of animals also thus follow the same law. 



The Particular Causes which determine the Difference 

 OF THE Directions of Evolution, and which also 



CONTRIBUTE TO BRING ABOUT THE DIVISION INTO SpECIES 



The ancestral tree of organic forms does not rise in 

 a straight line, but has ramose (forked) branchings. This 

 branching, which is also an important factor in the separation 

 of species, is due to (1) the direct influence of external con- 

 ditions, which, differing at each locality, act upon each stage 

 of evolution, and are able to divert the farther evolution from 

 the former direction; (2) the functional activity of the organism 

 in relation to the external world, which directly strengthens 

 characters in process of development by the exercise of them 



1 Compare TJeher das Variiren, etc., p. 454, separate edition, p. 218, where 

 examples are given. 



- Loc. cit. 



•' Compare also the researches of Weismann, subsequently to be more closely 

 considered, ou the markings of the caterpillars of the Sphingidje in his Shidien 

 zur Descendenztheorie, ii. : Ueber die letzten Ursachen der Transviutationen. 

 Leipzig, 1876. With regard to plants the force of the same law is shown — apart 

 from other facts — by the changes in the form of the leaves on one and the same 

 plant from the lower to the upper part. The upper leaves are often at a new 

 stage of evolution, the lower remaining at or passing through a lower stage — 

 the former indicate the new species, the latter ancestral species. 



