vi PREFACE. 



succeeding sections are occupied with the demonstration of these and other 

 generalizations, so far as practicable. In each of the sections some essays 

 will be found to be more, and others less, adapted to popular use. It 

 is believed that the general reader can select a sufficient number of articles 

 of minimum technicality to convey to his or her mind a sufficient idea 

 of the views set forth. In Part I, articles first, fourth, and sixth are of 

 this character. In Part II, the first, second, and third essays are the 

 least technical. In Part III, the first article is the most popular, although 

 the others are essential to an understauding of the doctrine of mechani- 

 cal evolution. In Part IV, all are sufficiently popular for the reader who 

 has some knowledge of mental science. 



A historical synopsis of the essays may be now given. The attempt is 

 made to point out the aim of each, with an indication of what may have 

 been new in its contents. They are taken up in the order of date of pub- 

 lication : 



II. The Origin of Genera. From the "Proceedings of the Philadel- 

 phia Academy of Natural Sciences" for October, 1868, and published sepa- 

 rately by the author early in 1869. In this essay the following doctrines 

 were taught : 



First, that development of new characters has been accomplished by an 

 acceleration or retardation in the growth of the parts changed. This was 

 demonstrated by reference to a class of facts, some of which were new, which 

 gave ground for the establishment of the new doctrine. 



Second, that oi exact parallelism between the adult of one individual or 

 set of individuals, and a transitional stage of one or more other individuals. 

 This doctrine is distinct from that of inexact parallelism which had already 

 been stated by von Baer. And that this law expresses the origin of gen- 

 era and higher groups, because — 



Third, they can only be distinguished l)y single characters when all their 

 representatives come to be known. 



Fourth, that genera and various other groups have descended, not from 

 a single generalized genus, etc., of the same group, but from corresponding 

 genera of one or more other groups. This was called the doctrine of 

 homologous groups. 



Fifth, the doctrine that these homologous groups belong to different 

 geological periods, and. 



Sixth, to different geographical areas, which, therefore, in some in- 

 stances, are. 



Seventh, related to each other in a successional way like the epochs of 

 geological time. 



Of these doctrines it may be observed that the first and second are now 

 the common property of evolutionists, and are recognized everywhere as 

 matter of fact. The names which I selected to express them have, however, 

 only come into partial use. The author beHeves that, although the doctrine 

 was vaguely shadowed out in the minds of students prior to the publication 

 of this essay, it had not previously been clearly expressed, nor been reduced 



