Alternation of Generations and Ontogeny. 217 
characters of the larva and imago of insects) and the special reaction 
to external conditions attained independently. The work of Klebs 
has made us familiar with external conditions which act as “releasing 
stimuli,” and so bring on the sexual or asexual phase in many 
Thallophyta; but between the way these stimuli act now and the way 
they have acted in evolution there is clearly a wide gap which can 
only be bridged by speculation. In the case of teleutospores the 
onset of autumn conditions appears to act as a stimulus in relation 
to the development of these structures, but apart from their influence 
on natural selection the effect of such condition on the origin of 
teleutospores is very problematical. Similarly the discovery of the 
conditions under which the sporophyte and gametophyte now arise 
will not necessarily throw light on the origin of the differences 
between the two generations. 
One does not wish to be unduly pessimistic but until we have 
further knowledge of the nature of the internal factors which work 
in development it would seem probable that the exact nature of the 
physiological relationship of the two generations will remain as 
inaccessible to direct proof as is the question of the actual history 
of the two generations. The latter problem has to be attacked on 
the grounds of “ historical probability based on comparative 
morphology” (to use the words of Professor Farmer) and it would 
seem that reliance has also to be mainly placed on the comparative 
method in dealing with the question raised by Dr. Lang. It is 
suggested that such evidence as is here brought forward points 
strongly against the “ ontogenetic theory,” and is in favour of the 
alternation being a part of the protoplasmic mechanism. 
In conclusion one may draw attention to the close similarity of 
the problems in relation to the metamorphosis of insects and those 
concerned in the alternation of generations in Archegoniatm. 
Information on the subject of this process has recently been 
brought together in a very interesting way in a small work by 
Deegener 1 . We have all degrees of difference between the larva 
and imago. Some organs are rudimentary in both, some are 
inherited by the imago from the larva, some are common to the 
larva and imago but follow a different course of development in 
each, etc. The larva has practically all the organs present in the 
imago but many organs present in the larva are absent in the 
imago ; hence the imago is probably phylogenetically older than the 
larva. The two generations are clearly homologous and there were 
] P. Deegener. Die Metamorphose der Insekten, s. 56, 1909 
(Teubner). 
