26 The Significance of Sex. [Jan. 
Spontaneous generation once accounted for the presence of the 
swarms of minuter forms of life, but scientific study, aided by the 
microscope, showed that these lower forms of life multiplied by 
methods obtaining with the higher forms, and the doctrine of 
spontaneous generation was, by Tyndall’s beautiful experiments, 
finally banished from the realm of the minutest infusorial life— 
to which, as a last resort, it had been restricted. But, with the 
establishment of the law that all living beings are derived from 
pre-existing forms of life, we also learned that another method of 
reproduction, the asexual (agamogenesis), was more widely used 
by nature than the sexual one, and increases in importance as we 
descend the organic phyla,—is, in fact, the foundation on which 
the latter rests, and out of which it has been evolved as a rare and 
expensive, but useful, link in generation. 
It is now a half-century since biology received its organon in 
the formulation of the ce//-doctrine. From this doctrine we must 
start in every biologic inquiry. Stated briefly and in the light of 
the present, it stands thus: The body of any of the larger plants 
or animals is a mass of minute units, called ce//s, that are organized 
in a complex way into different orders of higher units, or parts, 
known as żissues and organs. The unity or individuality of the 
organism is secured by the harmonious working together of its 
organs, like the parts of a mechanism, towards simple results for 
the good of the whole. All living beings, compared as to struc- 
ture (morphology), naturally fall into groups that are related like 
the branches of a tree (phylogenetic classification). At the roots 
we place the unicellular beings, then, as we reach the lowest and 
least subdivided branches, we find organisms represented by 
simple aggregations of cells like those which lower down live as 
independent beings ; and, as we rise along the phyla, such aggre- 
gates become more and more complex in organization. In the 
development (ontogeny) of an individual its organization ‘is es- 
tablished in the following manner. We start with a single cell, 
which produces an aggregation by continued self-division, and 
then the units differentiate into the tissues and organs, becoming 
successively more and more complex, so that the embryologic 
history—leaving out of consideration secondary or cenogenetic 
a modifications—is a repetition of the stages seen as we ascend its 
| phylum in the natural system. Such relations as these could 
| have beds established wir by the actual evolution of living 
