3 i4 THE POPULAR SCIENCE MONTHLY. 



Naturae, contains the names of the species new to science added 

 to our lists during the year of which it treats ; and in the record 

 of each year we find the names of two or three times as many as 

 are mentioned in the whole Systema Naturae. Yet the field shows 

 no signs of exhaustion. As these volumes stand on the shelf 

 together, it is easy to see that the later volumes are the thickest, 

 and that the record for the present year is the largest of all. The 

 additional species named and described in 1889 are more than ten 

 thousand. Moreover, what is true of the increase of knowledge 

 in systematic zoology, is even more marked in the case of botany. 

 Such, then, is the variety of life on the globe — a variety of which 

 Linnaeus and his successors had never dared to dream. 



And yet, great as this variety is, there are, after all, only a few 

 types of structure among all animals and plants — some three or 

 four or eight or ten general modes of development — all the rest 

 being minor variations from these few types. 



It is even true that all life is but a series of modifications of a 

 single plan ; for all organisms are composed of cells, the essential 

 element of which is always a single substance — protoplasm. All 

 are governed by the same laws of development, reproduction, and 

 susceptibility to outside influences. Unity in life is therefore not 

 less a fact than is life's great diversity. In whatever way we 

 account for the diversity, the essential unity must not be forgot- 

 ten. The bonds of unity among organisms constitute what the 

 naturalist calls homology. 



That these resemblances have some deep significance, no 

 thoughtful student of nature has ever doubted. What this sig- 

 nificance may be is the underlying question in that branch of 

 philosophy which has come to be known as evolution. 



In the present discussion I shall take for- granted that answer 

 to these questions which is associated with the name of Darwin ; 

 and, as a student of the relations and distribution of animals, I 

 firmly believe that no answer to these questions fundamentally 

 different from his will ever be possible. 



The essence of the Darwinian theory is this, that the various 

 species of the present day are all derived from pre-existing forms, 

 more or less unlike them ; that this derivation takes place through 

 the operation of natural laws — the law of heredity, the law of 

 response to external stimulus or environment, and the law less 

 clearly understood by which variations from ancestral types are 

 constantly produced; the "divine initiative" in the individual 

 which struggles against sameness and monotony. The constant 

 tendency of organisms to multiplication by geometric progression 

 in a world of limited extent, already apparently full, brings about 

 a constant struggle for existence among these organisms, and by 

 this struggle, we have the progressive adjustment of individuals 



