428 



GENERATION. 



It was then that the foundation was laid for 

 the discussion between Epigenesis and Evolu- 

 tion, the two theories of generation which have 

 more recently occupied the attention of men 

 of science, and which, as has already been 

 remarked, relate principally to the nature of 

 the formative process. Harvey and Malpighi 

 may be regarded as the first who endea- 

 voured, from the observation of facts, to 

 establish the general law of Epigenesis as 

 opposed to the older views of Preformation 

 entertained by the Ovists or Spermatists ; but 

 it was not till near the middle of the last 

 century that these opinions were opposed to one 

 another in a decidedly controversial manner. 



At that time Caspar Frederick Wolff* sup- 

 ported the system of Epigenesis by a reference 

 to observations on the minute changes of the 

 egg of the fowl during the early stages of 

 formation of the chick, while Haller and Bonnet 

 advocated the opposite opinion of Evolution. 



Wolff and those who followed his system 

 held that no appearance of the new animal is 

 to be found in the perfect impregnated egg 

 before the commencement of incubation, but 

 that when the formative process is established 

 by the influence of heat, air, and other circum- 

 stances necessary to induce it, the parts of the 

 foetus are gradually put together or built up by 

 the apposition of their constituent molecules. 

 Hallerf referred both to his own observations 

 on the chick and to a variety of collateral 

 arguments in support of the system of Evolu- 

 tion, holding that when the foetus makes its 

 appearance in the egg, it does so merely in 

 consequence of the enlargement or evolution 

 of its parts which pre-exist, though in an 

 invisible condition, in the egg. Bonnet]: carried 

 this theory further than any one else, but 

 trusting mainly to the observations of Haller 

 on the formation of the fcetus, he supported his 

 overdrawn views on highly hypothetical reason- 

 ing. Bonnet, in what is termed the theory of 

 Emboitement, held not only that the whole of 

 the parts of the foetus pre-exist in the egg 

 before the time of their appearance, but also 

 that the germs of all the animals which have 

 been or are to be born pre-exist from the begin- 

 ning in the ovaries of the female ; that the 

 genital organs of the first parents of any species, 

 therefore, contain the germs of all their pos- 

 terity; that these germs lie dormant in their 

 abode until one or more are aroused by the 

 exciting influence of the male; and that con- 

 sequently there is not in nature the new forma- 

 tion of any animal. 



We shall have occasion to shew in the article 

 OVUM that the most recent researches concern- 

 ing the mode of formation of the foetus in 

 birds, quadrupeds, and other animals, and 

 more particularly the microscopic observations 



* Theoria Generations, published as an Inaugu- 

 ral Dissertation at Berlin in 1759, and republished 

 in 8vo. in 1774. 



t Elementa Physiologiae, &c. torn. vii. Mem. 

 sur la formation du Cceur dans le Poulet. Lau- 

 sanne, 1753. Opera Minora, torn. ii. 



t Palingenesie Philosophique, Geneve, 1769 ; 

 also in his Considerations stir les Corps Organises. 



of Meckel, Pander, Baer, and Rathke,* have 

 shewn the theory of Epigenesis or super- 

 formation of parts to be much more consistent 

 with what is known from observation than the 

 theory of Evolution. In modern writings, 

 however, the term Development is, without 

 reference to theory, employed to denote the 

 mode of growth of the fcetus more frequently 

 than any other. 



We would further remark in relation to our 

 present subject that various names have at 

 different times been given by authors gene- 

 ralizing the phenomena of development to the 

 powers supposed to operate in the formation of 

 the young; as, for example, the Anima vege- 

 tativa, Nistis format ivus, Vis plastica, Vis 

 essential'iSj expansive, resisting, and vegeta- 

 tive forces. These terms can be considered 

 as little else than general expressions of the 

 fact that the foetus is formed and grows in the 

 egg, and are not more satisfactory expla- 

 nations of the cause of its formation than the 

 hypothesis of organic affinity is of the process 

 of assimilation in the adult animal. As the 

 knowledge of minute anatomy and physiology 

 has increased, and the accurate observation of 

 the process of developement has been more 

 extended, the number of such hypotheses has 

 gradually diminished. 



Thus the somewhat vague discussion as to 

 the relative probability of Epigenesis and Evo- 

 lution has led to the laborious and accurate 

 investigation of the various steps of the forma- 

 tive process or developement of the foetus, and 

 the conjectures as to the forces or causes which 

 give rise to the growth of the new animal have 

 fallen into comparative neglect; the erroneous 

 notions respecting the source of the germs of 

 male or female offsprings from one or other 

 ovary or testicle have been replaced by a more 

 satisfactory examination of the mode of deve- 

 lopment of the sexual organs in the early stages 

 of their advancement; and the inquiry as to 

 the share taken by one or other parent in the 

 process of generation has been pursued in more 

 modern times by the attentive investigation of 

 the functions of the male and female organs of 

 reproduction, upon the same principles that 

 guide the physiologist in his attempts to explain 

 any other class of functions of the economy. 



Recent writings on our subject are not, how- 

 ever, altogether free from vague hypotheses of 

 the same nature as the older theories of gene- 

 ration above mentioned. The mechanical 

 explanation of fecundation by the entrance of 

 the seminal animalcule into the egg has been 

 revived by one author; a second considers 

 all the changes of development as under the 

 influence of electro-magnetic currents ; and a 

 third explains the same changes by attributing 

 them to a spontaneous motive power and 

 organic affinitive properties of the molecules 

 of the ovum. 



It has been well remarked by Professor 



* After these observers may be mentioned Serres, 

 Prevost and Dumas, Dutrochet, Rolando, Purkinje 

 and Valentin, Coste, and others, as contributing 

 materially to the knowledge of this subject. 



