FACTORS AND MECHANISM OF EVOLUTION 55 



basis of this sort of species-making, must be added to our list 

 of evolution factors. 



Some other biologists, of whom the botanist Nageli, the 

 zoologist Eimer, and the paleontologist Cope are representa- 

 tives (all three of these men, however, having evolution theories 

 and beliefs distinct and peculiar to each), believe in what may 

 be called orthogenetic evolution. That is, that the lines of 

 descent are determined by the appearance of certain special 

 determinate lines or tendencies of variation or change, this non- 

 fortuitous and determinate variation being itself determined 

 by certain causes either (in Nageli 's belief) inherent in life, or 

 (in Eimer's belief) extrinsic to life but imposed upon it, as for 

 example the influence of climate, etc. So that orthogenesis or 

 determinate variation should also find a place in any list of 

 assumed evolution factors. 



While it is apparent that variation is ever present and also 

 apparent that heredity or the fact of likeness is always ever to 

 be relied on, the exact relationship or correlation of these two 

 evolution factors is not so apparent. That heredity often 

 preserves or perpetuates variations after they have occurred is 

 well proved, but it is also proved that some variations appearing 

 in the parent are not handed on to the parent's offspring, nor 

 indeed to any future generations of the line. And the general 

 answer to the natural query raised by this condition is that 

 variations which are congenital or blastogenic, that is, are 

 determined at birth for it (although they appear of course only 

 after development), are heritable (that is, will be passed on 

 from parent to offspring); but that variations or modifications 

 acquired during the lifetime of the individual, that is, those which 

 are impressed on it by extrinsic influences during its "growing 

 up " or development, will not be heritable. Thus such modifica- 

 tions in body parts as may be produced by use or disuse, or by 

 other functional stimulation or lack of it, changes caused by 

 mutilation or disease, etc., are believed by most biologists to be 

 non-heritable. Hence it is that only the congenital variations 

 are looked on by these biologists as of importance in the matter 

 of species-forming. Yet the whole pre-Darwinian evolution 

 theory of Lamarck was founded on the assumption that the 

 modifications in individuals due to use, disuse, and other func- 

 tional stimulation, in a word that all body change and adapta- 

 tion, all characters acquired during the lifetime of an individual, 



