&>OLOaY AND BOTAKY, MICEOSCOPY, ETC. S3 



growth of the world of organiBms under variable conditions. The 

 heterogeneous links of this growing chain of organisms persist as species. 

 Varieties and species are essentially nothing but groups of forms which 

 have remained at various stages of a progressive development,"^ The 

 stoppage of forms at various levels, the author terms Genepistasis — the 

 still-standing of the form. 



As to the special means which determine the difference in the direc- 

 tions of evolution, and cause division into Species, Prof. Eimer takes the 

 following six into account and discusses each in detail: — (1) direct ex- 

 ternal influence ; (2) strengthening through function or the reverse ; 

 (3) struggle for existence — an indirect influence ; (4) saltatory develop- 

 ment or sudden variations arising as the result of correlation in kaleido- 

 scopic fashion ; (5) " constitutional impregnation " or " conservative 

 adaptation " due to continued persistence under the same conditions ; 

 (6) sexual intermingling. 



Space does not permit a review of the succeeding chapters which 

 give part of the evidence. They discuss adaptation ; acquired charac- 

 ters ; disuse of organs, degeneration and panmixia ; the acquisition and 

 inheritance of intellectual characteristics ; the development of organs 

 and systems ; the laws of growth. Enough has been said to indicate 

 the standpoint of the author and the importance of his endeavour to 

 demonstrate more perfectly " the unity of organic nature." A second 

 volume of evidence and historical matter is promised. 



Divergent Evolution through Cumulative Segregation.*— In an 

 elaborate paper the Eev. J. T. Gulick follows up some previous commu- 

 nications, in which he has maintained that " separation without a 

 difference of external circumstances is a condition sufficient to insure 

 divergence in type." The abundance of technical and unique termino- 

 logy, combined with the intrinsic complexity of the inquiry, renders it 

 very difficult to present a brief summary without injustice to the patient 

 author. 



The importance of separation was suggested by a study of Sandwich 

 Island terrestrial molluscs. Under one set of external conditions diver- 

 vergence of type was observed to occur in a way which did not appeaif 

 to be explicable by Natural Selectioui The explanation seemed to the 

 author to lie in " a law rising out of the very nature of organic activities^ 

 a law of segregation, bringing together forms similarly endowed, and 

 separating them from their neighbours." It is this drawing of like to 

 like, in its manifold forms and influences^ which Mr. Gulick has set 

 himself to analyse. He does not raise the question of the conditions of 

 variation, but simply postulates a "frequency of deviation from an 

 average." Nor are the problems of direct environmental action, or of 

 hereditary transmission, at all discussed. The whole inquiry is con- 

 cerned with the forms and influences of segregation^ Mr. Gulick's 

 position differs considerably from Wagner's insistence on isolation, for 

 the latter depended solely on migration and geographical barriers, while 

 the separation and segregation dealt wdth by the author are much wider^ 

 His principle of segregate breeding is allied rather to Spencer's law of 

 segregation. The author differs also from Romanes, who has in his 

 " Physiological Selection " theory laid emphasis on the separating 



* Journ. Linn. Soc. (Zool.), xx. (1888) pp-. 189-274. Cf. criticism by A. B 

 Wallace in ' Nature,' xxxviii. (1888) pp. 490-1. 



