336 THE CELL 



another, were advanced up to the beginning of this century ; viz., 

 the theory of Preformation or Evolution and the theory of Epiyenesis. 



The theory of Preformation was embraced by such well-known 

 authorities of the 17th and 18th centuries, as Swamrnerdam, Mal- 

 pighi, Leeuvvenhoek, Haller, Bonnet (IX. 3), and Spallanzani (cf. 

 His IX. 14). They held the opinion, that the germ, as regards 

 structure, absolutely resembles the mature organism, and that 

 hence it must, from the very first, possess similar organs, which, 

 although extremely minute, must be in the same positions and 

 similarly related to one another. Since, however, it was impossible 

 by means of the microscopes at their command, actually to observe 

 and demonstrate these organs, which they assumed to be present 

 in the egg at the beginning of its development, they took refuge 

 in the theory, that certain parts, such as the nervous system, 

 glands, bones, etc., were present not only in a minute, but also in 

 a transparent condition. 



In order to render the process more comprehensible, the de- 

 velopment of the butterfly from the chrysalis, and the flower from 

 the bud, were quoted as examples. Just as a small bud of green, 

 tightly closed sepals, contains all the parts of the flower, such as 

 stamens and coloured petals, and as these parts grow in secret, and 

 then suddenly, when the sepals unfold, become revealed, so the 

 " Preformists " considered, that the minute parts, which are sup- 

 posed to be present in a transparent condition, grow, gradually 

 reveal themselves, and become perceptible to our eyes. 



Hence the old name of the " theory of Evolution or Unfolding" 

 in the place of which the more pertinent, intelligible, designa- 

 tion of the " theory of Preformation " has been adopted. For the 

 peculiarity of this doctrine, is that nothing is supposed to be 

 newly formed at any period of development, each part being 

 present or preformed from the beginning, and that, therefore, the 

 true nature of development or growth is denied. " There is no 

 new development," says Haller, in his Elements of Physiology; 

 " no part in the animal body is formed before the other ; all are 

 created at the same time." 



The theory of Epigenesis is directly opposed to the theory of 

 Preformation. Its chief supporter was Caspar Friedrich Wolff 

 (IX. 36), who lived in the middle of the 18th century. Tn his 

 important paper, entitled " Theoria Generations," published in the 

 year 1759 (Germ. ed. 1764), he enunciated the following axiom, 

 which was in opposition to the generally accepted dogma of pre- 



