BRITISH ASSOCIATION : ZOOLOGICAL ADDRESS. 351 



made up — of what '? Of vital units of a lower order, as several very 

 distinguished biologists would have us believe ; of physiological 

 units, of micellae, of determinants and biophors, or of pangenes ; all 

 of them essentially morphological conceptions ; the products of 

 imagination projected beyond the confines of the visible, yet always 

 restrained by having only one source of experience — namely, the 

 visible. One may give unstinted admiration to the brilliancy, and 

 even set a high value on the usefulness, of these attempts to give 

 formal representations of the genesis of organic structure, and yet 

 recognise that their chief utility has been to make us realise more 

 clearly the problems that have yet to be solved. 



Stripped of all the verbiage that has accumulated about them, the 

 simple questions that lie immediately before us are : What are the 

 causes which produce changes in the forms of animals and plants ? 

 Are they purely internal, and, if so, are their laws discoverable ? Or 

 are they partly or wholly external, and, if so, how far can we find 

 relations of cause and effect between ascertained chemical and physical 

 phenomena and the structural responses of living beings? 



As an attempt to answer the last of these questions, we have the 

 recent researches of the experimental morphologists and embryo- 

 logists directed towards the very aim that Hofmeister proposed. 

 Originally founded by Eoux, the school of experimental embryology 

 has outgrown its infancy and has developed into a vigorous youth. 

 It has produced some very remarkable results, which cannot fail to 

 exercise a lasting influence on the course of zoological studies. We 

 have learnt from it a number of positive facts, from which we may 

 draw very important conclusions, subversive of some of the most 

 cherished ideas of whilom morphologists. It has been proved by 

 experiment that very small changes in the chemical and physical 

 environment may and do produce specific form-changes in developing 

 organisms, and in such experiments the consequence follows so 

 regularly on the antecedent that we cannot doubt that we have true 

 relations of cause and effect. It is not the least interesting outcome 

 of these experiments that, as Loeb has remarked, it is as yet im- 

 possible to connect in a rational way the effects produced with the 

 causes which produced them, and it is also impossible to define in a 

 simple way the character of the change so produced. For example, 

 there is no obvious connection between the minute quantity of 

 sulphates present in sea- water and the number and position of the 

 characteristic calcareous spicules in the larva of a Sea-urchin. Yet 

 Herbst has shown that if the eggs of Sea-urchins are reared in sea- 

 water deprived of the needful sulphates (normally -26 per cent, 

 magnesium sulphate and -1 per cent, calcium sulphate), the number and 

 relative positions of these spicules are altered, and, in addition, changes 

 are produced in other organs, such as the gut and the ciliated bands. 

 Again, there is no obvious connection between the presence of a small 

 excess of magnesium chloride in sea-water and the development of 

 the paired optic vesicles. Yet Stockard, by adding magnesium chloride 

 to sea-water in the proportion of 6 grams of the former to 100 c.c. of the 

 latter, has produced specific effects on the eyes of developing embryos 

 of the Minnow (Fundulus heteroclitus) : the optic vesicles, instead of 



