Mutations and Evolution, 229 
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 (6) the fact that in Metapodius (Wilson 
1910) and Drosophila (Bridges 1916) 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. 
2 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. 
