THE PROGRESS OF ZOOLOGY 475 



The theory of special creation is that all animals were in the 

 beginning created by some omnipotent being. That of spon- 

 taneous generation holds that animals arise directly from in- 

 organic substances ; for example, ancient naturalists believed 

 that frogs and toads arose from the muddy bottom of ponds 

 under the influence of the sun, and that insects originated from 

 the dew. 



Darwin's book, The Origin of Species by Means of Natural 

 Selection, which appeared in 1859, placed the theory of organic 

 evolution on a firm foundation. At the present time practically 

 all zoologists believe that animals can arise only from preexist- 

 ing animals by reproduction, and that by changes of some sort 

 complex animals have evolved from simpler species. Argu- 

 ments in favor of this belief have been derived from the study 

 of comparative anatomy, physiology, embryology, classifica- 

 tion, geographical distribution, and of fossil remains of animals 

 that are found embedded in the earth's crust. At the present 

 time zoologists take for granted that evolution has occurred, 

 but are actively engaged in efforts to discover how it has taken 

 place. 



Gregor Mendel (1822-1884). One method of attacking 

 the subject of evolution is to study heredity ; that is, the study 

 of the similarities of and differences between parents and their 

 offspring. This is especially effective when animals or plants 

 of different kinds are bred together. At the present time the 

 foremost law of heredity is that discovered by Mendel (Fig. 324), 

 a monk who lived in an Austrian monastery. Mendel crossed 

 different kinds of peas and found that the offspring all re- 

 sembled one of the parents. When these offspring were inter- 

 bred, however, three fourths of their offspring resembled one 

 grandparent and one fourth resembled the other grandparent. 

 This and other facts discovered by Mendel have been found 

 to hold true for many animals as well as plants and constitute 

 what is known as Mendel's law. 



Pasteur (1822-1895). -- There are two kinds of science 



