DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION IOI 



and female elements that these two cells are in some way en- 

 dowed to carry the parental qualities. They are all that pass 

 from the old generation to the new. There are strong evidences 

 that the chromatic elements, or chromosomes, in the nuclei of 

 the male and female cells are the most important material struc- 

 tures by means of which transmission is effected. The chromo- 

 somes of the fertilized ovum are contributed equally by the male 

 and female elements, and they are the only structures in the 

 sperm and ovum which are apparently equal in amount. This 

 taken in connection with the fact that one parent does not seem 

 to have any more power than the other, on the average, to influ- 

 ence offspring, furnishes a basis for the belief that the chromo- 

 somes are the physical basis of heredity. 



Many good students of the subject believe that the cytoplasm 

 is, equally with the chromosomes, a carrier of heredity qualities. 

 They think that cytoplasm is responsible for those qualities 

 that both sexes have in common. That is, the cytoplasm con- 

 serves the character of the species. The chromosomes are held 

 to be more responsible for those qualities in which the individuals 

 of a species differ. Recent investigations seem to show that 

 the male and female chromosomes retain their distinctness in 

 large measure and are equally distributed to all the nuclei of 

 the developing embryo. 



132. Library Exercises. The student may increase his knowledge of the facts 

 of heredity by endeavoring to find answers to the following questions. What is 

 atavism, and what explanations have been offered for it? Do the male and female 

 seem, as a rule, to have equal power of transmitting their individual characteristics? 

 Cite some facts tending to show that the nucleus is especially concerned in trans- 

 mitting parental qualities; that the chromosomes are instrumental therein. What 

 are the essential features of the old " perf ormation " hypothesis to account for the 

 fact that an adult similar to the parent springs from an egg? Examine some of 

 the principal theories of inheritance: Darwin's "pangenesis;" Brooks' modifica- 

 tion of it; Weismann's "continuity of germ-plasm," etc. What is Mendel's law 

 of inheritance? 



133. Variability. Notwithstanding the fundamental like- 

 ness existing between parent and offspring, and among the off- 

 spring of common parents, no two individuals even among the 

 lowest animals are exactly alike. This fact of variation is only 

 less fundamental than the fact of likeness. Variation among 



