ELEMENTARY VITAL PHENOMENA 207 



We cannot go further into the special phenomena of the 

 individual development of animals and plants, and must refer the 

 reader to the detailed works of Haeckel ('91), 0. Hertwig ('90), and 

 Korschelt and Heider ('90), who treat embryology as an independ- 

 ent science. We must, nevertheless, glance at that important law 

 which, as has already been seen, prescribes a definite path to 

 individual development, namely, the fundamental law of biogenesis. 



Karl Ernst von Baer, the founder of embryology, discovered 

 that in the embryonic development of widely different forms of 

 animals, stages occur that appear strikingly similar; and after 

 Darwin's epoch-making labour Fritz Miiller ('64) expressed clearly 

 the fact that the developmental history of the individual is a short 

 repetition of the whole course of development which the corres- 

 ponding species has undergone during the development of the 

 earth. It was Haeckel's service to formulate more exactly the 

 fundamental law of biogenesis and emphasise the existence of a 

 causal relation between ontogeny and phylogeny. Haeckel ('66) 

 showed that individual development, or ontogeny, is only in 

 gross outline a repetition or palingeny of the racial develop- 

 ment or phylogeny, but that this repetition is frequently blurred or 

 falsified by the appearance of phenomena that are not present in 

 the phylogeny of the corresponding form and which, therefore, 

 he termed the phenomena of falsified development or cenogeny. 

 Hence, in the individual development of every organism, two 

 elements may be distinguished : first, the palingenetic phenomena, 

 which recapitulate in brief the racial development of the form in 

 question, and, second, the cenogenetic phenomena which have 

 arisen supplementarily by adaptation and have altered and blurred 

 the course of the palingenetic phenomena. 



The causal explanation of these facts lies in the two factors which, 

 as has been seen, control the whole development of organic life, 

 namely, heredity, which maintains form, and adaptation, which 

 changes it. 



The characteristics of an organism comprise more than those 

 which it shows at any single moment of its development or 

 as an adult animal. To them belong the whole sum of pe- 

 culiarities and changes which it has shown from its simplest 

 beginnings ; for the later characteristics do not represent any- 

 thing new and spontaneous, but proceed immediately and continu- 

 ously from the earlier ones. If, therefore, heredity conveys the 

 characteristics of the parents to the offspring, it must convey 

 to the latter, not only the characteristics possessed by the 

 parents at the moment of the production of the offspring, but 

 the whole sum of parental characteristics, and among them those 

 that the parents have shown during their development. Hence 

 the peculiar course of development that the parents have gone 

 through must be transmitted to the children, and the latter must 



