xxx SCIENCE PROGRESS. 



that he is going to be interested. And the perusal of the book will not 

 disappoint the reader's expectation. 



The author discusses the nature and the complex interaction of the 

 causes which operate in effecting the development and the diversity of 

 organic forms, and thus at once brings us face to face with problems no 

 less interesting to the morphologist than to the physiologist. Opening 

 with an account of the morphological and histological peculiarities of 

 tissues Dr. Hertwig proceeds to examine them from the point of view 

 of the functions they severally discharge. He critises acutely the 

 reasoning of writers who are ready to refer all the phenomena of life to 

 processes analogous to those exhibited by a machine, and points out 

 some of the insidious fallacies which underlie such comparisons. 



He insists strenuously on the fact that there are two distinct factors, 

 both of which modify the course followed by the constituent cells of a 

 developing organism ; first, the external, and second, the internal factors. 

 And he well brings out the point so vitally important for his own theory 

 of organised development, namely, that the very presence of a number 

 of cells in a multicellular individual must cause a reaction amongst 

 themselves, since each one is environed by all the rest. Of course this 

 was pointed out by Herbert Spencer many years ago, but it has not 

 always received the attention it deserves, and in fact it is deliberately 

 ignored in one of the most carefully worked out of the current theories 

 of heredity. It may be remarked in passing, that Prof. Hertwig 

 might have considerably strengthened his case for the actively modi- 

 fying effect of the environment by a reference to Klebs' recent work 

 on algae and fungi, in which the various factors which determine this 

 or that course of development have been successfully distinguished and 

 isolated. 



The internal factor, however, it must be confessed, still remains as 

 great a puzzle as ever. We are really totally ignorant of the nature of 

 the causes which determine the development of this or that animal or 

 plant out of apparently similar primordia. Nor are we possessed of 

 any definite knowledge as to the nature of the processes which we term 

 correlative. Of course it is clear that a correlation between the various 

 constituent cells of an organism must and does exist, but beyond this 

 we know very little. In the animal world, the experiments of Driesch, 

 Wilson and others on embryos have considerably broadened the basis 

 of facts, but, without in the least desiring to detract from their im- 

 portance, they cannot be said to have done much more. It is, more- 

 over, not unlikely that the production of "identical twins" in the 

 human species may be due to the separation and independent develop- 

 ment of the halves of a fertilised ovum which normally would have 

 developed into one individual, the two halves sinking their separate 

 individuality in producing the single child. 



In the vegetable kingdom the zygotes of some fungi and algae may 

 either grow out directly into single embryos, or they may give rise to 

 several embryos apiece if the first formed cells separate from one another 



