On Reproduction, Animal Life Cycles. 
77 
only a limited time, and in the end a portion of the parent is left 
as a residuum that dies. 
All three forms of reproduction of the Protozoa are, accordingly, 
methods by which the organism frees itself of harmful waste products, 
therefore modes of excretion on the part of the offspring. 
On turning to the multicellular animals we find a distinction be- 
tween portions that persist from generation to generation, the germ 
cells, the links in the chain of individuals, and portions that die, the 
body cells, a fruitful idea founded by Gal ton , 1 Nussbaum , 2 Brooks , 3 
and particularly Weismann 4 and numerous later publications.' But 
unicellular organisms are not entirely deathless and exactly com- 
parable with the germ cells of the multicellular, for as we have just 
seen a portion of the Protozoan individual dies in the act of reproduc- 
tion, and that portion, the residuum, is to be compared with the soma 
of a multicellular organism. Therefore we can say that the Protozoan 
differs from the Metazoan merely in not having its germ and soma 
disposed in different cells, and there is in reality no such radical dis- 
tinction between the two as is generally contended. 
In Metazoa there are two kinds of reproduction: sexual, which is 
generation by means of an egg cell whether that egg cell be fertilized 
or not; and asexual generation. Germ cells have much in common 
with Protozoan spores, and sexual reproduction is more or less, though 
not wholly, comparable with sporulation. Of the germ cells only 
the egg cells are reproductive, since they only have the power of 
division; the spermatozoa are simply fertilizing elements carrying 
the hereditable energies of the father, for they have lost the power 
of division and but for their hereditable qualities might be ranked 
with specialized body cells. 
Now to understand the physiological cause of sexual reproduction, 
we must answer the question, which is by no means academic and 
incapable of empirical solution: what is the nature of the process 
that occasions the death of a portion of the individual, and what is 
the connection of this death with reproduction? Perhaps the best 
substantiated view, though I cannot say with whom it first originated 
or how general its acceptance may be, is that natural death of the 
individual results from self-poisoning. The waste products of metabo- 
lism, some of them toxic, accumulate in the tissues until there results 
a true intoxication of the latter. We may try to transcribe this into 
a little more definite physiological phrase: death follows on account 
a A theory of Heredity, 1875, Contemporary Review, 27. 
2 Zur Differenzierung des Geschlechts im Tierreich, 1880, Arch. mikr. Anat., 18. 
3 The Law of Heredity, Baltimore, 1883. 
4 Ueber die Vererbung, 1883, Jena. 
