SOME CONSEQUENCES OF EVOLUTIONARY RELATIONSHIP 377 



so named because they have a small second pair of upper incisors hidden 

 behind the first pair. It now seems evident, on the basis of skull structure, 

 reproductive organs, and teeth, that the two groups are only distantly 

 related and their similarities due to analogous modification. The pikas, 

 hares, and rabbits are now assigned to a separate order, the Lagomorpha. 



None of these gnawing mammals possesses a pouch for the young. In 

 all of them the coracoid bone is reduced to a projection from the scapula, 

 the vagina is single, and the development of the young occurs wholly 

 within the body of the mother, the fetus being attached to the wall of 

 the uterus by a placenta. The same characteristics extend through the 

 species of many other orders. These include the moles and shrews (In- 

 sectivora), flying lemurs (Dermoptera), bats (Chiroptera), lemurs, 

 monkeys, apes, and man (Primates), sloths (Edentata), carnivores 

 (Carnivora), elephants (Proboscidea), manatees (Sirenia), and the hoofed 

 mammals (Perissodactyla, Artiodactyla). All of these are included with 

 the Rodentia and Lagomorpha in a single subclass, the placental mam- 

 mals, or Eutheria. 



Thus it continues. All the groupings of the species of the plant and 

 animal kingdoms are based on the same principle. Taxonomists do not 

 create these groupings, which exist by virtue of the evolutionary relation- 

 ships between species. Instead the student of taxonomy is trying to 

 determine what the natural groupings are and to discover how they are 

 related to one another. The fact that he is able to establish a system of 

 classification based upon degrees of homologous resemblance and capable 

 of a graduated and detailed grouping of all organisms is both a demon- 

 stration and a result of the fact of evolutionary kinship. 



ADAPTIVE RADIATION AND ADAPTIVE CONVERGENCE 



When we discussed homology and analogy at the beginning of this 

 chapter, we were concerned chiefly with the criteria by which these two 

 classes of resemblance might be distinguished. We then saw how these 

 concepts gave meaning to the facts of comparative morphology, em- 

 bryology, physiology, and classification. Now let us see how, in terms of 

 adaptation to environment, they enter into the two contrasting evolution- 

 ary phenomena known as adaptive radiation and adaptive convergence. 



The adaptation of organisms to particular modes of life has two aspects. 

 On the one hand we see the great perfection of many adaptive structures 

 and behaviors ; on the other we note rather frequent cases in which species 

 seem imperfectly adapted to their environments or fail to show adapta- 

 tions that would appear desirable. Both aspects of adaptation are under- 

 standable if we remember that evolution consists in the modification of 

 organisms. If a species has a structure or process or behavior which is 

 suitable material for adjustment to new conditions, a new adaptation 



