6 NATURE SKETCHES IN TEMPERATE AMERICA 



but others are not so clearly separated and are sometimes 

 divided into subgenera, which is the lowest definable group 

 of species. 



In zoology the phylogeny, or descent, of only a portion of 

 the groups of animals has been definitely ascertained, but 

 the diagram from Galloway gives an approximate numerical 

 relation of the groups as they exist on the earth. (See diagram 

 facing first page of text.) 



Struggle foe Existence 



In the past organic forms were recognized as species when 

 intermediate forms were absent. When these connecting 

 forms were living or were found in the fossil state, naturalists 

 usually regarded the whole series as varieties and named all 

 the members of it as belonging to the same species. In the 

 naming of species they designated cases where connecting 

 forms have not been observed. 



In the systematic divisions of plants and animals in groups 

 higher than species, such as genera, families, and orders, grada- 

 tions have been traced which show these divisions should 

 be regarded as conventional. 



Although the pre-Darwinian students of nature recognized 

 gradation among the groups of organisms, in the case of 

 species they supposed them to be the rriiraculously created 

 units of organic life, in contrast with the present conception 

 that they are the product of natural evolution or gradual 

 transmutation. Wallace states that so far as observation goes 

 "every species has come into existence coincident both in space 

 and time with preexisting and closely allied species." Geo- 

 logical evidence shows that extinct species which now occur 

 only as fossils on any given geological area, resemble the 

 species still living upon that area. 



It is obvious that the vast hordes of animals have a great 

 struggle for existence, and it is to explain the philosophy of 

 how these animals maintain themselves upon the earth, and 

 the causes operating in perpetuating their lives, that the the- 

 ory of natural selection is propounded. Darwin says:' "All 

 organic beings without exception tend to increase at so high 

 ' " Origin of Species." 



