DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION IO3 



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 "performation" 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. State Mendel's laws of 

 inheritance? What did Galton, De Vries, and Johanssen contribute to the 

 subject? 



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. While members of the same 

 species agree more than they differ, the fact of variation is less 

 fundamental only than the fact of likeness. Variation among 

 animals appears to depend upon two sets of considerations: 

 (i) the physical and chemical instability and changeableness of 

 the protoplasm of which animals are so largely composed, and 

 (2) the diversity and stimulating effect of the environment in 

 the broadest sense. Through the interaction of these two influ- 

 ences, even if all individuals were alike at the start, it would 

 only be a question of time until the offspring derived from them 

 would present noteworthy differences. Such differences would 

 tend to increase with the lapse of time. This is the more true 

 in proportion to the degree in which variations are capable of 

 being transmitted under the influence of heredity. So far as 

 we can tell organisms may vary to any amount if we give time 

 enough. 



134. The Part Played by the Environment in Producing 

 Variation while not completely understood must be recognized 

 as very real. Even though much stress must be put upon the 

 hereditary complexity and instability of protoplasm as the 

 source of variations, it is evident that the external conditions 

 serve as stimuli to produce the changes on the inside. For 

 example, it is a matter of common observation that the quantity 

 and quality of food greatly influence not merely the rate of , 



