314 The Realm of Nature CHAP. 



changes ; as the continents have by long and gradual degrees 

 come to their present form, and are still undergoing altera- 

 tion, so also living creatures display a progressive evolution. 

 The classifications of plants ( 396) and of animals ( 397) 

 are ascending scales, showing in each group a more com- 

 plex structure and organs more distinctly set apart for 

 special purposes. Amongst animals, for example, the pro- 

 tozoa have no organs at all ; the single cell acts as a whole in 

 every function. In the echinoderms, eyes and a separate 

 stomach appear ; in the arthropoda, limbs adapted for walk- 

 ing ; and an internal skeleton connected to a backbone, and 

 supporting the framework of the body, is only found in 

 the vertebrata. Similar progressive advancement is to be 

 found within each group, and even in the same species in- 

 dividuals vary so much that a regular gradation may often 

 be traced into other species making it difficult to draw 

 the dividing line. Transition types, such as Archasopteryx 

 ( 349)5 a bird partly resembling a reptile, and the 

 Australian duck-bill, which although a mammal has a beak 

 like a bird and lays eggs, connect the different classes of 

 animals or of plants. When this regular order of succes- 

 sion from lower to higher forms in plants and animals 

 became apparent to biologists they were convinced that 

 different species had not been created separately in different 

 places, but had gradually developed in the course of ages 

 from a common parent form. The late Charles Darwin 

 and Mr. A. R. Wallace almost simultaneously framed a 

 theory to account for organic evolution the gradual un- 

 folding of the progressive design of plant and animal life ; 

 and the period of most rapid advance in modern biology 

 dates from the publication of Darwin's Origin of Species in 

 1859. The original views of Darwin and Wallace are 

 gradually being modified as new facts are encountered and 

 the general principles of evolution stand out more clearly. 



403. Heredity and Environment. Darwin explained 

 the origin of different species of living creatures by the 

 two great influences of heredity or likeness to parents and 

 environment or surrounding circumstances. As a rule the 

 young of plants and animals resemble their parents, but no 



