2 THE STUDY OF IX SECTS. 



The diversity of iorms of animal life is much greater than is com 

 monly supposed. A competent authority has estimated that there 

 are now living on the earth more than one million species of animals. 

 And these are merely the surviving descendants of immense series 

 of beings that have existed in past geological times, the remaining 

 tips of a great genealogical tree, of which many twigs and branches 

 have perished. 



The common figurative use of the word tree in this connection 

 expresses well the convergence of the lines of descent toward the 

 common ancestor from which existing forms have descended. But 

 in one respect it may be misleading. If an ordinary tree be ex- 

 amined, the tip of one branch will closely resemble that of any other 

 branch of the same tree. But in this figurative genealogical tree 

 we must imagine a very different state of affairs. Here the law of 

 growth is constant change ; each branch grows in its own individual 

 way; and each twig of each branch bears fruit peculiar to itself. 

 The changes, however, are gradual ; and thus the tips of closely-con- 

 n ct(d twigs will be similar though not identical; while the tips of 

 two branches that separated early in the growth of the tree will be 

 very different. 



It is the effort of the systematist, one who studies the classification 

 of animals and plants, to work out the relations which exist between 

 the various tips of the genealogical tree. This study when carried 

 to its fullest extent includes not only the study of existing forms of 

 life, but also the study of those that have perished, the trunk-forms 

 from which existing forms have descended. This, however, is a very 

 difficult matter; and as yet only the beginnings of the Natural 

 Classification have been made. See pp. 199 to 204. 



If we accept this theory of descent, now almost universally ac- 

 cepted by naturalists, it is evident that when we take into account 

 ail the forms of life that have existed we cannot classify animals into 

 well-marked groups; for as the modification in form is gradual, series 

 of connecting links have existed between any two forms that might 

 be selected. 



But practically the student that confines his attention to the 

 study of living forms can classify these forms into more or less well- 

 marked groups, for many of the connecting links have perished; in 

 fact, the groups of living animals and plants are so distinct that it ?s 

 only in recent years that naturalists have come to understand tne 

 blood-relationship referred to above. 



We find that the Animal and Vegetable Kingdoms a.e made up 01 

 a vast assemblage of individuals^ each the offspring ci parents similar 



