EXISTING ORGANISMS— CLASSIFICATION 25 



pupa a resting stage through which transition from the larval to 

 the very different adult structures is accomplished, and the adult 

 is primarily the reproductive stage. 



What Are Species? In the face of such diversity of form 

 within many species, it is impossible to lay down definite criteria 

 for the limitation of this unit of classification. The ability of indi- 

 viduals to produce fertile offspring when mated has been adopted 

 by some scientists, l)ut it has been shown that not only do many 

 closely related species cross, but in some cases they produce fertile 

 offspring which maintain themselves within the range of variation 

 represented by their diverse parents. Criteria of morphology fail 

 when great diversity occurs, and rigid delimitation on this basis 

 covers a latitude in single colonial species greater than the differ- 

 ences between some obviously distinct species. Differences of 

 physiology are difficult to judge, yet species exist between which 

 no other distinctions are known. We are forced to the conclusion 

 already briefly expressed, that there are specific entities in nature, 

 although the conditions of their existence are variable. Since the 

 species is the unit whose occurrence evolution proposes to ex- 

 plain, this very instalMlity is significant. Those species which are 

 variable, and apparently undergoing change, seem about to give 

 rise to several different species, while those which are fixed within 

 relatively narrow limits seem to be more definitely established. 

 By referring to the past we see that still other species have come 

 and gone, apparently after passing through a period of senility 

 characterized by inability to adapt themselves to changing condi- 

 tions. In this, again, palaeontology clarifies our understanding 

 of the condition of modern species, although the modern species 

 alone show that distinctness of kind is relative. 



Major Groups. Beyond those relationships which enable us 

 to group individuals together as species, we find other points of 

 similarity which indicate broader associations. Our robins and 

 bluebirds, for example, have definite structural characteristics in 

 which both differ from the crow and the hawks, so we call both 

 thrushes, yet the thrushes and hawks are more closely related to 

 each other than to our domestic animals, because they are birds. 

 Step by step these resemblances proceed through groups of in- 

 creasing extent, each based on more fundamental characters than 

 the one below it, and conse(iuently embracing a wider range of 

 species. The system of classification developed by Linnaeus and 



