220 
THE PRINCIPI.ES OF BIOLOGY. 
into cells. This is just wliat we do find; moreover, every 
individual, however complex, if produced by gamogenesis, is 
at first but a simple cell, and naked protoplasm is sometimes 
found to enter into the life-history of organisms otherwise 
highly developed. 
A unicellular organism is an aggregate of the first order. 
These are mostly of small size; but some such, as among 
Algfe, Codium, Bryopsis and Botrydium,* and among Fungi, 
Mucor and Peronospora,* reach a considerable size. The 
first step towards an aggregate of the second order is mani¬ 
fested in the fact that, in some unicellular organisms that 
multiply by simple fission or budding, the daughter-cells remain 
for a time attached to each other, or to the parent cell, instead 
of separating, as others do, at once. The Diatoms furnish 
an instructive series. Some, such as Bacillaria, simply slide 
upon one another’s longer edge in an irregular fashion; others, 
as Isthmia, remain attached by the corners only; while some 
remain permanently connected into more or less tree-like 
forms. The Desmidieae furnish a somewhat similar series of 
more and more perfect aggregation. In Yeast, again, we have 
an instance where cells continue in contact with one another, 
although so slight and immaterial is the union that the 
slightest force suffices to break it. In Sarcina and in most 
Confervse we find a greater coherence of the component cells 
and a greater degree of individuation of the compound. The 
same type is still further developed and completed under 
various forms and by various methods in the Lichens, the 
Mushrooms, and the higher Algae. A perfect instance of a 
spherical aggregate of the second order is furnished by Yolvox, 
where the cells are united in a definite form and mass of 
only one degree of composition, so long as it is not engaged 
in the process of multiplication. 
Thus cells, which are aggregates of the first order, are 
compounded into fronds or thalli, which are aggregates of the 
second order. Each cell of the thallus has lost its individu¬ 
ality and become merely a part of a whole ; that is to say, 
it is integrated with others, and its life is merged in the life 
of the compound. But still there is no sudden transition ; a 
complete series can be traced from loose fortuitous aggrega¬ 
tions of cells, like those of yeast, up to the final and complete 
integration which constitutes the frond of a Laminaria. 
Those branches of the vegetable kingdom which are pre¬ 
dominantly of the second order of aggregation are called 
Tliallogens or Tliallophyta. 
* These are what are called in the text Hydrogastrum and Botrytis 
respectively. What is now known as Botrytis is not a unicellular 
fungus. 
