Adaptability and Alterability of Structures 125 



development, that they can adapt their form to new 

 conditions, and that their specific form is not determined 

 by special determinants in the nucleus but by external 

 stimuli." 89 



The stomach of the tern, which ordinarily feeds 

 on fish, is lined by a soft mucous membrane. If 

 one feeds it with wheat for a few weeks its stomach 

 develops a superficial horny coat, its musculature is 

 strengthened and it takes on the character of a gizzard. 90 

 If these stomachs belonged to two varieties of the same 

 species, Weismann would have no hesitation in attribut- 

 ing the diversity to special, and thus different, determi- 

 nants which, as the facts show, do not really exist. 



Loeb has demonstrated that the colored design of 

 the yolk sac of a fish embryo (Fundulus) is not in itself 

 predetermined, but depends upon the distribution of 

 blood vessels. The pigment cells are at first distributed 

 uniformly but when the circulation of the yolk sac is 

 established, they migrate toward the vessels, attracted 

 probably, as Loeb supposes, by a chemical substance in 

 the blood, and give rise thus to a definite design. Graf 

 has likewise recently demonstrated that the color designs 

 of the leech are not themselves inherited, but that they 

 depend upon the disposition of muscle fibers in which 

 the amoeboid pigment cells lie. It would be absurd, 

 concludes Wilson, to imagine in all of these cases a 

 special series of determinants for each individual color 

 design. 91 



89 Oscar Hertwig: Zeit- und Streitfragen der Biologic. I. Pra- 

 formation oder Epigenese? P. 48 49. 



90 Delage: L'heredite etc. P. 604. 



91 E. B. Wilson : The embryological Criterion of Homology. Biol. 

 Lect. at the Mar. Biol. Lab. of Wood's Holl ; Summer Session 1894. 

 Boston, U. S. A., Ginn. 1896. P. 116. 



