102 ZOOLOGY 



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 interaction 

 of these two influences, 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 varia- 

 tions 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 

 growth of the body but the size and quality of the adult organ- 

 ism as well. Life would be impossible without food, oxygen, 

 water and suitable temperature. Any variation in these con- 

 ditions at once has its effect upon the organism. Experiment 

 shows that the varying degrees of salinity of the water may be 

 accompanied by striking individual differences of form in certain 

 marine animals. Caterpillars of certain butterflies placed in 

 boxes lined with differently colored papers develop pupae with 

 colors harmonizing with those of the boxes containing them. 

 Colors in various animals are intensified or changed by special 

 foods or by changed temperature. In general it may be said 

 that changes in any of the conditions important to animal life 

 produce some change or variation in the body of those animals 

 subjected thereto. Since this is true, it becomes inevitable that 

 the various individual animals on the earth are differentiated 

 from each other somewhat as was seen to be the case with the 

 cells and tissue of which the individual itself is composed. The 



