2 14 Alternation of Generations and Ontogeny. 
A study then of the relations of the generations in Dictyota 
and the Uredineae, together with a brief examination of animals, 
like insects and the Foraminifera, which show two body forms, 
together with evidence obtained from other organisms which show 
complicated life-cycles, all point to the conclusion that the morpho¬ 
genetic stimuli are mainly internal, and that, as in the ontogeny of 
the single body each stage is determined by the preceding stage, so 
in the more complicated life-cycle each phase is in the main 
determined by the preceding phase. It is suggested also that a 
comparison of the germ-cells of Bryophyta and Pteridophyta 
supports this view. 
Of course it is not contended that external factors do not play 
any part in ordering the life-cycle . 1 Organisms which pass their life 
under conditions which differ markedly from time to time must clearly 
make use of such “orientating stimuli.” Since, however, it can be 
shown that cyclic correlation plays so important a part (both where the 
conditions are constant and inconstant), it would seem that such a 
theory of the relation of the generations should hold the field in 
any given case, until there is direct evidence to the contrary, or 
until we have some clearer general ideas of the way external 
conditions affect development. 
The differences impressed upon the germ-cells are not conceived 
as unalterably fixed. The germ-cells (and somatic cells) contain all 
the characters of the organism and under special circumstances 
characters which are normally latent may be brought out. A change 
of external conditions may in the sporophyte cause an alteration 
in growth relations so that the leaf-cells, which normally have all 
but leaf-characters latent, may produce shoots. Similarly a change 
of conditions from the normal may affect the internal factors and 
cause an alteration in the life-cycle by bringing out certain 
characters at unusual phases in the cycle. 
It has been urged against the view that the germ-cells are 
different that it complicates the question of heredity. But it has 
been shown that in other cases there is a clear difference among the 
germ-cells, as in the Uredineae, so that the complication must be 
accepted. How much more complex than in the Archegoniatae 
must be the transmission of characters in the insects with a 
1 The internal factors are naturally related to external factors. 
There is some evidence, for example, that with the increase 
of the size of the larva of insects there is partial asphyxiation, 
and that this want of oxygen plays some part in bringing on the 
changes which lead to the development of the imago. 
