1 5 ° 
V. H. Blackman. 
have ended either in conjugation or death, and in one case to carry 
the organism beyond the 620th generation. 
The replacement of conjugation by what must be a definite 
chemical stimulus is certainly a very striking observation and 
strongly suggests that during the temporary union the two individuals 
supply one another with chemical substances of which a lack has 
arisen in the organisms. The observed advantage of exogamous 
over endogamous conjugation would then be explained, as individuals 
of different broods grown under different conditions would be likely 
to have different needs, and by a process of interchange be able to 
supply each the other’s wants. Calkins is of opinion that conju¬ 
gation is much less common in nature than under artificial conditions, 
for every rainfall would bring into a pond in which Pammoecium 
might be living a change of dissolved substances and so obviate the 
necessity for conjugation. 
These observations are in agreement with the well known 
observations of Loeb and others, in which normal eggs were made 
to develop for a time in a truly (but artificial) parthenogenetic way, 
by placing them in certain solutions, by alterations in temperature 
etc., or by placing them in a filtered extract of spermatic fluid 
{Winkler). There can thus be little doubt that one of the results 
of fertilisation is a chemical effect which brings about a change 
in the metabolism of the cells which unite. 
Although in the majority of living organisms the process of 
fertilisation occurs sooner or later in the series of cell-divisions, yet 
the need for such a process cannot be considered as an inherent 
property of protoplasm, for such organisms as the Bacteria, Cyano- 
phycese, and Mycetozoa seem to be completely agamic. Either 
their protoplasm is of such a nature that it does not become 
exhausted by a prolonged series of divisions or else a rejuvenescence 
is brought about in some other way, either by long periods of rest or 
by change of environment. In a few species also of the higher 
animals, males have never been observed and the only method 
of reproduction seems to be that of so-called parthenogenesis; also 
in some few Angiosperms, and perhaps in Chara crinita, the life- 
cycle is carried on without any form of gametic union (syngamy), 
although sexual organs are present. 
That the process of union of gametes from different individuals 
brings about a mingling of different ancestral characters is, of 
course, sufficiently clear, but whether this process or that of proto¬ 
plasmic rejuvenescence is to be considered as the essential, primary 
