METHODS OF PHYSIOLOGICAL RESEARCH 17 



mass of facts into a closed and intelligible whole is always 

 stimulating and fruitful, and this explains the immense authority 

 and powerful influence which Haller exercised in the develop- 

 ment of physiological investigation. His own physiological re- 

 searches, however, while very conscientious and exact, as, e.g., 

 those upon the respiratory movements and the theory of irritability, 

 contain no epoch-making discoveries, and some of them even had 

 the misfortune to play an obstructive role in the further develop- 

 ment of the science. This is especially true of two doctrines which 

 he advocated the so-called theory of preformation, and the theory 

 of irritability. 



The theory of preformation (theory of incasement) arose in 

 connection with the microscopic observations upon the development 

 of the ovum which were made in the seventeenth century. When 

 it was seen how from a single small egg a complete animal was 

 developed by the gradual maturing of one organ after another, the 

 idea arose that all organs appearing in the course of development 

 and, in brief, the whole animal, are preformed or already enclosed as 

 such within the egg, and are made visible to the eye only by a 

 process of growth and unfolding ; that, therefore, the human egg 

 or, as some believed, the spermatozoon, is a minute but a completely 

 formed homunculus. The necessary consequence of this idea was 

 the assumption that at the creation of the world all coming 

 generations were contained, already preformed, in the egg of each 

 animal. The preposterousness of this view led a young physician, 

 Caspar Friedrich Wolff (1733-1794) to maintain a new theory in 

 opposition to that of preformation. Wolff's " theoria generationis" 

 which later became the basis of all our modern ideas of the 

 development of organisms, denied incasement and put in its place 

 epigenesis. This asserted that all organs of the body are formed 

 one after another in the course of development, in other words, that 

 they originate as entirely new parts and have never pre-existed as 

 such in the egg. Haller could not accept the idea of epigenesis, 

 but opposed it energetically ; and, supporting the dogma of pre- 

 formation with his whole authority, he retarded progress in the 

 doctrine of animal development for more than half a century. 



Haller's theory of irritability influenced the development of physi- 

 ology in a somewhat different manner. Haller's own researches in 

 this direction were experimental and very exact, and materially 

 advanced the general theory of irritability ; but they were mis- 

 interpreted in various respects and extended by his followers, and 

 formed the chief starting-point of a doctrine that confused all 

 physiology down to the middle of the present century, and even 

 now emerges again here and there in varied form. This is the 

 doctrine of vital force. The fact of the irritability, or the direct 

 excitability, of muscles had been emphasized by the earlier iatro- 

 physicists, especially by Glisson (1597-1677). Haller took up the 



