172 
POPULAR SCIENCE REVIEW. 
and a complete animal is as it were divided into separate crea- 
tures, or a number of animals are so intimately connected that 
the associated life seems to swallow up the individual life. 
Among the hydroid polyps, for example, two kinds of budding 
are conspicuous ; one adding new members to the colony, much 
as first leaves and branches grow on a tree ; and the other acting 
according to wfiat is technically called “ discontinuous gemma- 
tion,” that is, producing buds that drop off and become distinct 
organisms, differing from the immediate parent so as to appear 
creatures of another family. Besides this, true eggs are formed 
by the action of male and female elements. Zoologically all 
the products of a single egg are considered as one individual 
notwithstanding the number of forms that may be assumed, 
and the intermediate steps that may take place before another 
true egg is produced, and a fresh zoological individual started 
upon a new comer. 
Professor Clark begins his essay by a protest against that theory 
of individuality which assumes that the separate Medusoid or 
jelly-fish forms taken by the reproductive organs of certain 
hydroid polyps are individuals in a higher degree than the polyps 
are ; and he remarks that “ in the discussions of late years 
upon the individuality of the lower, compound, colonial denizens 
of the water, the main points at issue have always been to 
determine whether a certain form was, on one hand, an indi- 
vidual , either in the highest sense, or one of several inde- 
pendent individuals which constitute a colony ; or, on the other 
hand, an organ which formed only part of an individual.” In 
the highest individuals each separate body contains all the 
parts and organs necessary for the due performance of a com- 
plete cycle of life functions, while in lower forms certain organs 
occupy distinct bodies, and would be taken for complete indi- 
viduals if their relation to other forms belonging to the same 
individuals was not known. Professor Clark refers to an obser- 
vation of Lereboullet to show that even in the vertebrates a 
tendency towards dualism exists, as he found an instance in a 
fish’s egg of two heads and Wo tails proceeding from one centre 
of development. In many of the lower creatures the tendency 
is towards multiplicity — an extension of duality. Physiolo- 
gists use the term bilaterality to describe the symmetrical 
arrangements of parts in a man for example — the growths on 
each side of a median line corresponding with each other. In 
the development from an ovum directive forces are exerted, not 
only to produce this bilateral arrangement, but also to tend to 
what he terms “ antero-posteriority and dorso-ventrality,” terms 
easily understood. These actions sufficiently resemble what the 
physicist calls polarity to justify the use of that word in desig- 
nating them ; but it must be borne in mind that scarcely any- 
