I06 FOOT-NOTES TO EVOLUTION. 



the germinal protoplasm, and that, therefore, develop- 

 ment is, in the words of Professor Whitman, " a function 

 of organization." 



Inheritance and variation are general terms which 

 include a great many different kinds of phenomena, 

 many of which seem to be due to entirely different fac- 

 tors. A great many phenomena of inheritance seem to 

 be due entirely to extrinsic forces, but a more careful 

 inquiry always reveals the fact that they are invariably 

 due to the reaction of certain extrinsic causes on a per- 

 fectly definite living structure. As examples may be 

 mentioned the following: 



{a) The tiger-like striping of the egg of Fundulus, 

 which is very characteristic and would certainly be re- 

 garded as an inherited character, has been shown by 

 Loeb* to be due entirely to the position of the blood 

 vessels of the blastoderm. The pigment cells are at first 

 uniformly distributed, but when the blood vessels are 

 formed they gather around them, probably through 

 chemotropic action, and thus the characteristic banded 

 appearance is produced. Graf f has since shown that 

 the colour patterns of leeches are produced in the same 

 way. It is not necessary, therefore, to assume that the 

 colour patterns in these cases are specifically represented 

 in the germinal protoplasm; it may even be that the 

 position of the blood vessels is not so represented, but 

 there must be some ultimate cause back in the germinal 

 plasm itself which determines the series of causes which 

 finally produces the colour patterns. In short, this fea- 

 ture, like most others, was predetermined from the be- 

 ginning. 



* Jacques Loeb. Some Facts and Principles of Physiological /^H 

 Morphology. Biological Lectures, 1893. ^^ 



f Arnold Graf. Ueber den Ursprung des Pigments und der 

 Zeichnung bci den Hirudineen. Zool. Anzeiger, No. 468, 1895. 



