88 Mutations and Evolution. 



covers the whole under surface of the leaf. Similar developmental 

 changes take place in a number of other species. 



Balfour thinks this developmental modification is in relation 

 not to a climatic change in the habitat of the species, but to the 

 differences in environmental conditions as regards, light, moisture, 

 heat and air currents encountered by the leaves of the young plant 

 near the soil and of the older plant at a higher level. He points 

 out that a higher temperature and a more rapid metabolism 

 (subserved by anthocyanin) are important at first, while control of 

 transpiration (subserved by tomentum) is important later. 



The above case could not be regarded in itself as evidence of 

 recapitulation, but it serves to show how indubitable recapitulatory 

 phenomena shade into those which have only a physiological or 

 ecological rather than an ancestral significance. 



Summarizing the data of recapitulation in plants, we may say 

 that recapitulatory characters are found chiefly (1) in the seedlings 

 of Gymnosperms and some Angiosperms, (2) in the terminal stages 

 of gametophytes, (3) in wood structure ; but as a rule they have 

 been lost from the ontogeny through the cellular development 

 becoming direct. If the antithetic theory of alternation of 

 generations be correct, however, then a large part of evolution has 

 been concerned with the gradual development of characters which 

 were originally organismal and have become in some measure 

 recapitulatory. 



Recapitulation in Animals. 



Important and significant as are the indubitable cases of 

 recapitulation in plants, the phenomenon is much more prevalent 

 in animal development. This may perhaps be connected with the 

 fact that the animal in development may be said to have greater 

 power over its cells owing to their thinner walls and greater 

 plasticity. 1 A striking phenomenon in the cleavage of animal eggs 

 is the mutual readjustment of the cells with relation to each other 

 which goes on after each cleavage. 2 In this way the forces of the 



1 A number of facts indicate that in some respects animals have greater 

 powers of regulation than plants. One need only mention (a) phenomena of 

 metamorphosis in which tissues may break down and be used again in the 

 building up of new structures ; and (b) the fact thrtit in Metapodius (Wilson 

 1910) and Drosophila (Bridges 19U5) the presence of one or even several extra 

 chromosomes (duplicates of one member) in the nuclei produces no external 

 alteration; while the duplication of one chromosome in CEnothera produces 

 striking external differences. The recent studies of Rosenberg (1918) on 

 Crepis indicate that here also the duplication of a pair of chromosomes alters 

 the external features of the plant. 



1 It is a well known fact that cleavage itself is a rhythmic process, 

 in which a period of simultaneous nuclear division alternates with a period of 

 nuclear growth, and this rhythmic alternation is accompanied by a physiological 

 rhythm in CO, production, permeability, etc. 



