334 EMBRYOGENESIS IN PLANTS 



latter's adult state, or show a progressive deviation from the ancestral 

 ontogeny. The first is a theory of parallelism ; the second is like one 

 of von Baer's laws {see below). In contrast to Haeckel, Miiller based 

 phylogeny on ontogeny; for it is by modifications during the process 

 of development that descendants, in their adult state, come to diff'er 

 from their adult ancestors. The contemporary view is that phylogeny 

 is the result of ontogeny (Garstang, 1922; de Beer, 1951): no case is 

 known in which the evolutionary modification has taken the form of an 

 addition of a new terminal phase to the final phases of some alleged 

 ancestral form. Where a critical attempt has been made to prove the 

 theory of recapitulation, as has been done in zoology, the theory has 

 been found to be invalid. As de Beer (1951, p. 51) says, there 'is 

 embryonic similarity and repetition of characters in corresponding 

 stages of the ontogenies of ancestor and descendant, which reveals the 

 affinity between different animals, but supplies no evidence of what the 

 adult ancestral form was like.' As early as 1828 von Baer, from a 

 study of animal development, had reached the following important 

 conclusions — known as the laws of von Baer (after de Beer, 1951, p. 3). 



1. In development from the egg, the general characters appear 

 before the special characters. 



2. From the more general characters the less general and finally 

 the special characters are developed. 



3. During its development an animal departs more and more from 

 the form of other animals. 



4. The young stages in the development of an animal are not like 

 the adult stages of other animals lower down on the scale, but are Hke 

 the young stages of those animals. 



Some points from de Beer's discussion of this field of research may 

 be briefly indicated here. Since modifications in ontogeny are due to 

 changes in hereditary factors, a phylogenetic sequence is the result of a 

 ssquence of modified ontogenies. Evolution is brought about by 

 significant qualitative and quantitative genetical changes. Factors 

 determining new characters, which may appear at all stages in the 

 ontogeny, may, during further changes, be accelerated or retarded in 

 their time of action, sometimes with momentous morphological 

 consequences. Characters which are present in the early ontogeny, and 

 provided they are not too specialised, may through paedomorphosis be 

 important in the evolutionary process in that large structural changes 

 are produced without loss of plasticity. {Paedomorphosis connotes the 

 production of phylogenetic effects by the introduction of youthful 

 characters into the line of adults; paedogenesis is the process by which 

 the appearance of embryonic or early ontogenetic characters is retarded 

 until the adult stage.) The conception and evidence of paedomorphosis 



