518 



SCIENCE. 



[N. S. Vol. VIII. No. 1£ 



Biogenesis.' A large part of the volume, be- 

 ginning with the eighth chapter, is taken up 

 with the consideration of the first foundation- 

 stone of the theory of Biogenesis, viz. : specific 

 and direct action of the environment in the in- 

 dividual development. The factors of develop- 

 ment are divided into external and internal; 

 the first being defined (p. 75) as ' the different 

 kinds of relationships to the external world 

 with its numerous forces. ' The internal factors 

 are of two sorts, viz. : ' the relations of a cell to 

 all other cells of the same aggregate,' and ' the 

 properties and organization of the sexual cells 

 and their derivatives; the internal factors in the 

 stricter sense.' It would seem likely to us to 

 conduce to more clearness if the first subdivi- 

 sion of the internal factors were reckoned as 

 external factors, as, indeed, they are from the 

 standpoint of the organization of the cells. 

 The author discusses this (p. 75), but thinks 

 otherwise. 



Under the head of ' External Factors of Or- 

 ganic development ' are discussed: gravity; 

 centrifugal force; mechanical effects of traction, 

 pressure and tension; light, temperature, chem- 

 ical stimuli; more complex stimuli; organic 

 stimuli, such as grafting, transplantation of 

 tissues, reactions between the embryo and 

 mother, and telegony. These subjects are 

 illustrated with many examples drawn from 

 botany and zoology indiflferently. Under the 

 head of ' Internal Relations of Organic Devel- 

 opment ' we have these subjects: the correla- 

 tions of cells during cleavage; the correlations 

 of organs and tissues in later stages of develop- 

 ment and in the adult; chemical correlations; 

 mechanical correlations; the phenomena of re- 

 generation and of heteromorphosis; and, finally, 

 different conditions and modifications of the 

 cells in the adult, such as hypertrophy, atrophy, 

 metaplasy, hyperplasy and necrosis. The mere 

 reading of such a menu is enough to make 

 one's mouth water ; but it must be said that 

 most of this part of the volume is pure 

 description ; and the facts are set too much in 

 one light, so that the author apparently loses 

 sight, at times, of the importance of the true 

 internal factors contained in the organization of 

 the cell. However, a great quantity of ex- 

 tremely valuable material is here gathered to- 



gether for the first time, and it would be un- 

 gracious to quarrel with the point of view, 

 considering the service to the teacher. 



The second foundation-stone of the theory is 

 the doctrine of the inheritance of acquired 

 characters. The chapters which illustrate and 

 attempt to substantiate this theory are, in many 

 respects, the most interesting of the entire 

 volume. It is here that we find a considera- 

 tion of ' the most important category of causes 

 in the process of development,' viz., the inner 

 factors in the narrower sense. Inheritance de- 

 pends on continuity ; " but the great problem, 

 so differently answered in the various theories, 

 is not the continuity of life as such, which is a 

 fact of experience, but the methods and means 

 by which the continuity on which the per- 

 sistence of a species depends is handed on from 

 member to member of a series of generations." 

 It is impossible to give here a detailed account 

 of Hertwig's views on the mechanism of the 

 inheritance of acquired characters ; but, having 

 rejected the transmission of mutilations and of 

 disease, he sums up thus : ' ' Changes in the 

 entirety of an organism produced by alteration 

 of any function during the life of the individual 

 induce, if lasting, alterations in the cells com- 

 posing the organism, especially in that sub- 

 stance which we have called the bearer of the 

 characteristics of the species" (that is, the idio- 

 plasm contained in the nucleus). "Conditions 

 of the entire organism are thus translated into 

 heritable properties of the cell, in a different 

 material system. The hereditary substance of 

 the organism is thus enriched by a new link, a 

 new possibility, which again becomes manifest 

 in the development of the next generation ; 

 thus the new individual now reproduces, to a 

 greater or less extent, from the germ or from 

 inner causes, the properties acquired by the 

 parents in their lifetime through reaction to the 

 environment." He seeks to render the difii- 

 culty, of understanding how this translation is 

 possible, less alarming by an analogy, follow- 

 ing Hering, between this and the capacity of a 

 nerve-cell to store up impressions and repro- 

 duce them again through memory. In both 

 cases the process can be reduced to its most 

 general formvila by saying : " External causes 

 exercise influences on a complicated organic 



