II. INTRODUCTION. 9 



immense number of specific and individual peculiarities of living beings. The 

 variation consists in the difference of the relative quantities of the indispensable 

 conditions required and supplied, in addition to a modifying influence of light, 

 probably of electricity, and possibly of some other but yet unknown agency. 



The absence or presence of light is a highly important modifying condition to 

 those indispensable to life. With the solar light, we find the green plant, which 

 constitutes the basis of life with most terrestrial animals. Without it, the green 

 plant and its dependent animals could not exist, but -another race, now represented 

 by certain cryptogamia, and the animal denizens of dark caverns, might inhabit 

 the earth. 



A species of plant or animal may be defined to be an immutable organic form, 

 whose characteristic distinctions may always be recognized by a study of its his- 

 tory. 



Any species may present individual forms not characteristic ; for all, in the pro- 

 gress of development and course of life, are liable to modification within definite 

 limits, which cannot be transcended without cessation of action. The original 

 proposition is, however, not affected, for no one has ever been able to demonstrate 

 the transmutation of one species into another. 



The most ordinary and extensive modifications of species from the characteristic 

 type are presented by arrests of development. Hence, the necessity of studying 

 the history or cyclical course of a species in order to be capable of always recog- 

 nizing it. 



A modification of condition beyond the range of specific life-action, must, necessa- 

 rily, result in the extinction of the species. 



The study of the earth's crust teaches us that very many species of plants and 

 animals became extinct at successive periods, while other races originated to occupy 

 their places. This probably was the result, in many cases, of a change in exterior 

 conditions incompatible with the life of certain species, and favorable to the primi- 

 tive production of others. But such a change does not always satisfactorily explain 

 the extinction of species. 1 



Probably every species has a definite course to run in consequence of a general 

 law ; an origin, an increase, a point of culmination, a decline, and an extinction. 

 Within this course there may occur, under the influence of ordinary circumstances, 

 cycles of temporary increase and diminution, until, finally, the entire machine of 

 life of the species runs down. 



The historical period of man is too short to ascertain with certainty whether 

 such a view be correct, but it appears to be favored by analogy. The power of re- 

 production is limited in each individual. Plants may be reproduced to an incalcu- 

 lable extent by cuttings, but ultimately the power to reproduce in this manner 



1 Thus, there are numerous instances of species of animals which have become extinct, and their place 

 supplied by others so closely allied, that it is difficult to comprehend how the exterior conditions for their 

 existence should be so different; as in the case of the Equus prlmiyenim, E. Amcricanus, &c., which have 

 given place to the E. caballus, the Bos primiijcniuts, whose place is supplied by the -Bos taurus, the Bison 

 latifrons by the Bison Amcricanus, &c. 



