feb. 1899] HERTWIG' S THEORY OF BIOGENESIS 153 



lioux, Poulton, etc.), and of the "epigenetics." (Hertwig, Driesch, Morgan, 

 etc.), in which he satisfactorily disproves Weismann's fundamental assump- 

 tion of the " qualitative " (erbungleich) division of the idioplasm of the 

 nucleus, but yet admits that such an idioplasm must exist, and so paves 

 the way towards an explanation of his " Theory of Biogenesis." Develop- 

 ment, according to Hertwig, depends on two sets of factors : (a) external \ 

 (f>) internal. Under the first he groups the various effects of environment, 

 and discusses at length the influence of such forces as gravitation, stress, 

 and pressure in the formation of organic tissues. Much space also is 

 devoted to the influence of light, temperature, and various chemical substances 

 on developing animals and plants. Among the internal factors Dr. Hertwig 

 deals chiefly with the correlation existing between the cells themselves 

 and between the organs they compose. These chapters form perhaps the 

 most important part of the book, for although it is generally admitted 

 that correlation must play an enormous part in ontogeny and phylogeny, 

 yet in text-books it is rare to find more than a few trite instances quoted, such 

 as the deafness of blue-eyed white cats, etc. 



We have also a mass of information with regard to the latest results 

 of experimental embryologists as to the phenomena of regeneration. After 

 discussing the many pathological changes undergone by cells, Dr. Hertwig 

 finally deals with such controversial points as the inheritance of acquired 

 characteristics, and the part played by the idioplasm of the cell itself, and 

 concludes with a sketch of the way in which his theory has gradually been 

 evolved. It may be briefly summarised as follows : — 



(1) Every cell has its own specific characteristics. 



(2) The hereditary material which one cell passes on to another is lodged 

 in the nucleus. 



(3) The invisible specific distinctions of an ovum give rise to the visible 

 specific distinctions of an adult organism solely by multiplication of cells, and 

 their actions and reactions upon one another. 



(4) The differences of cells are not produced by their receiving different 

 portions of the hereditary substance. All receive like shares, but do not 

 respond alike to external and internal stimuli. The fate of a cell largely 

 depends on the exact position it holds with regard to its neighbours. 



(5) Except in the lowest forms of life, each cell, as development proceeds, 

 gradually loses the power of reproducing the whole organism, but the germ 

 cells are exceptions to this rule. 



(6) Besides possessing hereditary substance, a cell, if it is to develop 

 into an organism, must possess the power of responding to certain external 

 conditions. Unless these latter are present development is impossible. 



In reviewing this book, all that has been possible to do is to give a bare 

 outline of Dr. Hertwig's methods and general aim, and we must leave 

 the reader to draw his own conclusions. Many, no doubt, will be satisfied with 

 the results arrived at. All will congratulate Dr. Hertwig on the completion 

 of his work, albeit some may be more than ever convinced that the end 

 of this century sees us still in almost complete ignorance of the causes of 

 organic development. M. D. Hill. 



LOST LINKS SUPPLIED. 



The Last Link. Our Present Knowledge of the Descent of Man. By 

 Ernst Haeckel. With Notes and Biographical Sketches by Hans 

 Gadow. 8vo, pp. 156. London : Adam and Charles Black, 1898. 

 Price 3s. 6d. 



About half of this little book, which seems dear for its size, is occupied with 

 the address which Professor Haeckel gave at the Congress of Zoologists at 



