54 Elementary Biology. 



stage exhibiting, in a greater or less degree, a likeness to 

 extinct or now existing organisms of a lower grade of organ- 

 isation. 



SECTION III. DISTRIBUTION. 



It may have been noted that in the preceding sections 

 we have answered three out of the four questions which 

 may be asked concerning every living thing. Morphology 

 answers the question, What is it? Physiology answers the 

 question, How does it live ? Phylogeny answers the ques- 

 tion, Whence came it? What was its origin ? One query 

 yet remains, Where is the organism found ? An answer to 

 that query is given by the section which treats of the 

 distribution of organisms. 



Any given group of animals may have a distribution in 

 space, or a geographical distribution, and also a distri- 

 bution in time, or a geological distribution. In the former 

 case the habitat and distribution over the earth's surface of 

 the members of the tribe is taken account of, in the latter, 

 note is taken of the occurrence of remains of the same forms 

 as fossils in the strata of the earth's crust. 



SECTION IV. CLASSIFICATION. 



It has been already explained that organisms widely differ 

 in the structure of their different organs. Hence we have 

 varying degrees of likeness and unlikeness among organisms, 

 and a starting-point is thus afforded us for classifying them. 

 Classification consists essentially in the grouping together 

 of like and the separation of unlike forms, not merely in 

 their adult condition, but after taking into account their 

 entire life-histories. Classifications may be either natural 

 or unnatural. By an unnatural classification is meant 

 one based on superficial or apparent resemblance, whilst a 

 natural classification is one based, not only on accurate 

 morphological investigation, but also on the story of relation- 

 ship with other organisms told by ontogeny. 



