On Reproduction, Animal Life Cycles. 
87 
continued and close juxtaposition of the elements of a pair must be 
of the nature of a true conjunction process, like that of two Protozoa, 
and it is to be regarded as the final and one of the most important 
parts of the fertilization process . 18 The conjugants must exert some 
influence upon each other, with probably an interchange of substance. 
This process may have the result that Maupas 19 argued to follow from 
conjugation in Ciliates, namely, a rejuvenation of the conjugants, 
and notwithstanding recent objections that have been urged to such 
a conclusion. For by such conjugation the chromosomes seem to be 
stimulated into new activity, which finds an expression in the energy 
of the subsequent growth period. It may even result that not only 
new growth energies become started in that way, but also a re-estab- 
lishment of the perfect power of excretion. Therefore this might be 
still another means, to be added to the three previously mentioned, 
by which the germ cells maintain themselves against intoxication. 
On proceeding to the somatic cycles proper, or rather to life cycles 
in the full meaning of the term, we would maintain the following 
theses. First, that in the Metazoa reproduction by the agency of 
single egg cells is more primitive than any other kind of generation. 
Second, that the condition of separated sexes is more primitive than 
the hermaphroditic one, the latter being not absence of sex but the 
union of two sexes in one individual. And third, that it is the more 
primitive process for the egg to be fertilized, in other words, that 
amphigony is more primitive than parthenogenesis. The foundation 
of these theses has been undertaken by me elsewhere . 20 In this book 
also the relations of the different kinds of reproduction and life cycles 
are more fully considered than can be done in the present place. 
The cycle from the egg to the final mature stage may be either 
continuous or discontinuous. 
Continuous are such life cycles that are complete within the bounds 
of one individual,, and where all stages are of more or less uniform 
habit. The simplest conceivable form of such a cycle would be the 
direct one, when each stage from the egg onwards is a closer approxi- 
mation to the adult condition and where there is no turning from a 
straight path. But it is doubtful whether any of the life histories 
known to us exhibit quite such simplicity. In most, if not in all, 
cases continuous life cycles are indirect, which happens when some 
younger stage shows a divergence from the immediate path to the 
adult, this may be visualized by an angular line, but a line without 
“This explanation was first suggested in my paper, “A Study of the Chromo- 
somes of the Germ Cells of Metazoa, 1901,” Trans. Amer. Phil. Soc., 20. 
19 Le rejeunissement karyogamique ehez les Cilies, 1889, Arch. Zool. exper. 
gener. 
20 The Analysis of Racial Descent in Animals, New York, 1906. 
