LECTURE IV. 



V A R I E T Y IN LlFE. 



Variety everywhere. "Nature likes to know her 

 creatures." Commonness the cloak of variety. The 

 green cloak which covers the brown earth the shield 

 under which millions of organisms, brown and green, 

 carry on their life work. 



Meaning of Species. A single kind of living object. 

 " Species are the twigs of a tree, disconnected from its 

 parent stem. We name and arrange them arbitrarily 

 in default of ability to reconstruct the whole tree in 

 accordance with nature's ramifications."- Coues. 



Linnaeus ; Systema Naturae, 1758 : Four thousand 

 species of animals. 



Zoological record 10,000 each year now added. 

 Each volume thicker than the one before it. Number 

 of species of animals now known, high in the millions ; 

 no one knows how far. Still more of plants. Ex- 

 tinct hosts. All these species phases of change. Vari- 

 ety within species ; no two individuals alike. No one 

 ever matched two clover leaves. Immense range of 

 individual variation. The variations due to : (1) In- 

 nate tendencies, the "unseen powers" within. (2) 

 To stimulus of surroundings, the " unseen powers" of 

 environment. (3) To double parentage, the "unseen 

 powers" of heredity. (4) Unlikeness of germ cells of 

 same individual. 



