LESSONS IN QEOLOGI. 379 



in almost every system of philosophy. They were matters of 

 speculation among the East Indians, the Arabs, the Jews, the 

 Greeks and the Schoolmen, and they have been discussed by later 

 philosophers, as Hobbs, Descartes, Locke, Kant, Hegel and 

 others. But the honor of bringing the doctrine before the world 

 in such a clear and forcible way as to command universal atten- 

 tion belongs to Charles Darwin and Russel Wallace, whose first 

 publications were in 1858 and 1859. 



One of the first steps toward this theory is the observed unity 

 of all forms of life, as shown by the fact that plants and animals 

 have about the same chemical composition; that plant and ani- 

 mal protoplasm appear to be identical; that the germinal vesicle 

 and sexual reproduction are similar in each; the difficulty in dis- 

 tinguishing between the lower forms of animals and plants; that 

 plants and animals are cellular in structure; that plants and an- 

 imals, as individuals, develop from a bit of structureless proto- 

 plasm to a complicated organism, each growing by the simple 

 multiplication of cells; and the fact that animals and plants are 

 affected in much the same way by physical environment all of 

 which with many other things point toward the unity, the con- 

 tinuity of life on the earth; plants and animals possibly develop- 

 ing from a common germinal stock, plants through fixed forms, 

 animals through motile forms. 



Another is the fact that offspring inherit some of the charac- 

 teristics of the parents, but not all. A number of children may 

 be similar to their parents and to each other in many respects, 

 the family features and traits may be well marked, yet each has 

 his pecmliarities, differing from all the others in some particulars. 

 This fact is expressed in the phrase, " heredity, or descent with 

 variation." Variation in size and activity is common among 

 men and domestic animals, and among wild animals as well, va- 

 riations in size sometimes amounting to as much as one fourth 

 of the average size of the species. There may be variations in 

 the length of legs, breadth of wings, size of fins and in other respects. 



Then the fact that a pair of the slowest breeding animals, or 

 plants if unchecked, would stock the earth to repletion within 



