VI PREFA CE. 



tematized into a science of embryology ; and as we also dis- 

 cover the grand features of the evolution of species and races 

 and kinds of organisms, life begins to assume the proportions 

 of one of the fundamental forces in the world. When con- 

 sidered from this point of view the question what causes the 

 evolution of organisms seems as impertinent as what causes 

 the motion of the celestial spheres. The answer to both is 

 the same. 



That the form and functions of successive organisms should 

 be accurately adjusted to their organic and physical environ- 

 ments is no more surprising than that the size and weight of 

 the revolving planets should be accurately adjusted to the 

 orbits in which they swing ; but once grant that the systems 

 are in motion, and it is not reasonable to suppose in either 

 case that at any point in the succession of phenomena misad- 

 justment should occur which would require any hypothetical 

 selective force to put them right again. Evolution thus becomes 

 one of the fundamental expressions of life force, requiring no 

 theory to support it, but calling only for investigation to re- 

 veal its laws ; and it is in geological biology that we find the 

 direct evidences of the course of its operation. But evolu- 

 tion is not all of biology, and therefore sufficient illustration 

 of their respective phenomena has been borrowed from physi- 

 ology and embryology to present a comprehensive view of all 

 the three great factors of organic life, viz., growth, develop- 

 ment, and evolution. 



A few of the chapters are somewhat technical in their 

 language, and deal with particulars of slight interest to those 

 unfamiliar with the nomenclature of natural history. These 

 chapters may be omitted by readers willing to take the 

 author's statements without verification. Such persons may 

 omit the purely geological part of the book by passing di- 

 rectly from Chapter I to Chapter V, where the discussion of 

 the biological problem begins. The more technical passages 

 are Chapter II; Chapter IV, except the summary at the 

 close ; pages 98 to 1 10 of Chapter V ; all but the summary of 

 Chapter VII ; the latter parts of Chapters XII and XIII ; and 

 the fine print of Chapters XVIII and XIX. The remainder of 

 the book, although occasionally expressed in scientific terms. 



