94 ZOOLOGY. 



nuclei of the male and female cells are the material structures 

 by means of which transmission is effected. The chromosomes 

 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 have any 

 more power, on the average, to influence offspring than the 

 other furnishes a basis for the belief that the chromosomes are 

 the physical basis of heredity. Recent investigations tend to 

 show that the male and female chromosomes retain their dis- 

 tinctness and are equally distributed to all the nuclei of the 

 developing embryo. 



128. Library Exercises. The student may increase his knowledge of 

 the facts of heredity by endeavoring to find answers to the following ques- 

 tions. 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 trans- 

 mitting their individual characteristics? Cite some facts tending to show 

 that the nucleus is especially concerned in transmitting parental qualities; 

 that the chromosomes are instrumental therein. What are the essential 

 features of the old " preformation " 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' 

 modification of it; Weismann's "continuity of germ-plasm," etc. What 

 is Mendel's law of inheritance? - , , 



129. 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 

 animals appears to depend upon two sets of considerations: 

 (i) the physical and chemical instability of the protoplasm of 

 which animals are so largely composed, and (2) the diversity 

 of the environment in the broadest sense. Through the inter- 

 action of these two influences, even if all individuals were alike 

 at the start, it would only be a question of time until the off- 

 spring derived from them would present noteworthy differ- 

 ences. Such differences would tend to increase with the lapse 

 of time. This is the more true in proportion to the degree in 



