336 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



of beginning students. So that we believe it well to postpone, 

 as we have done in this book, any special consideration of the 

 subjects associated with animal evolution until the student has 

 had some personally acquired experience in the study of the 

 other phases of zoology. 



Evolution a Fact. Although there is much discussion of the 

 causes of evolution there is practically none any longer of 

 evolution itself. Organic evolution is a fact, demonstrated 

 and accepted. The many important open questions about 

 evolution concern the factors that originate and guide it. 

 Evolution means the blood relationship of organisms, their 

 descent from common ancestors, the origin of kinds by the 

 transformation or modification of other already existing kinds. 



Evolution was not discovered by Charles Darwin nor by 

 Lamarck, whose names are, however, the greatest of all those 

 associated with it. It grew as a conception and a belief slowly 

 through all the years from the times of the Greeks until it 

 received its two most positive announcements and its two most 

 plausible explanations from Lamarck and Darwin. Darwin, 

 especially, presented such a full and convincing statement of 

 the facts that prove the reality of evolution that he put 

 evolution for all future time into our accepted understanding 

 of nature. Hence to Darwin belongs the greatest merit that 

 attaches to any name connected with the history of evolution. 



The Causes, or Factors, of Evolution. Evolution has two 

 conspicuous conditions of animal (and plant) life to explain; 

 first, all the different kinds of animals, and, second, all the 

 various adaptations or special fittings of these kinds to their 

 environment, so that they may live successfully as individuals 

 and persist as species. The factors or causal agencies pro- 

 posed to explain evolution must, therefore, to be acceptable, 

 explain not only the origin of species but their adaptations. 

 The factors proposed are many. Most conspicuous among 

 them are those called variation, heredity, selection and 

 segregation. 



Variation and Mutation. No two animals in the world are 

 alike. Whether widely related or so-called "identical twins," 

 products of a single egg, there are differences or variations be- 



