918 
Proceedings of the Boyal Society 
nutrition is relatively lese active and katabolism is maximised, 
the formation of flowers indicates the appearance of sexual repro- 
duction. 
(c) Alternation of Generations . — The alternation between tape- 
worm head and proglottides, between fixed hydroid and swimming 
bell, between leafy fern plant and inconspicuous prothallus, and the 
like, are familiar facts, which have, however, been studied almost 
exclusively on their morphological side. Even on superficial in- 
spection, however, of any of those numerous cases, where an 
asexual form alternates with one or more dimorphic sexual genera- 
tions, it is evident that we have here to do in two generations with 
what is often so obvious in one — the familiar antithesis between 
nutrition and reproduction. A consideration of the physiological 
distinctions between the asexual and sexual generations, shows that 
the former is the expression of favourable nutritive conditions 
resulting in vegetative growth, or at most in asexual multiplication, 
■while the latter is conditioned by less propitious circumstances. 
Just as a well-nourished plant may continue propagating itself by 
shoots and runners, and just as an Aphis in artificial summer may 
for years reproduce parthenogetically, so a hydroid with abundant 
food and otherwise favourable environment may be retained for a 
prolonged period vegetative and asexual, while dearth of food and 
otherwise altered conditions evoke the appearance of the sexual 
generation. That the tapeworm head, or the encysted scolex before 
it, should in a plethora of nutriment remain asexual, while the pro- 
glottides, further from the sources of supply, &c., exhibit sexual 
reproduction, conforms to what we have already seen in regard to 
the relation of the two processes to one another and to the condi- 
tions favouring anabolism or katabolism. The contrast between the 
deeply-rooted, well-expanded fern plant and the weakly-rooted, 
slightly-exposed prothallus, is obviously that between an organism 
in conditions favourable to the continuance and preponderance of 
anabolic processes, and that of an organism in an environment 
where katabolism is, at an early stage, likely to gain the ascendant, 
the former therefore is asexual, the latter sexual. A survey, in 
fact, of the conditions and characteristics of the two sets of forms 
inevitably leads us to regard the asexual generation as the expression 
of predominant anabolism, and the sexual as equally emphatically 
