510 LIFE-HISTORIES 
of agriculture and hygiene. Moreover, a peculiar theoretical 
interest attaches to thallus-plants from the belief that they 
include representatives of the forms from which all higher 
living things have evolved. Since the highest differentiation 
of the plant-body in thallophytes is for the most part a more 
or less elaborate lobing, the name lobeworts becomes a 
significant English equivalent for the group. 
_ In our survey of thallophytic types, several classes were omitted 
as involving unnecessary complication. Nevertheless, it is hoped 
that the student has gained some idea of the evolution of thallo- 
phytes which may be useful to him in further study. As helping 
further to a general conception of this multifarious division let us 
briefly review the main lines of development along which we may 
reasonably suppose the reproductive and the vegetative system of 
lobeworts to have been evolved. 
With ali normal creatures it is as if the controlling purpose of 
life were the production of well-endowed offspring. Organic evolu- 
tion seems thus to present a series of attempts to find the best ways 
of achieving this purpose under all possible conditions of existence. 
In the most primitive organisms, typified by Chrodécoccus, there 
is no differentiation into repreductive and vegetative parts, for the 
entire individual becomes the offspring by fission. Growth in a way 
prepares for fission, but it may also be regarded as a preliminary 
part of the reproductive process. Hence we have here what may 
be called vegetative reproduction in its simplest form. It is a method 
of propagation admirably adapted to uniformly favorable condi- 
tions, such as these little water-plants enjoy; and, so long as favor- 
able conditions prevail, vegetative reproduction is the promptest 
possible way of taking fullest advantage of them. We therefore see 
it retained with more or less modification by organisms which have 
developed other methods as well, as for example in desmids. When 
the offspring remain attached, as in Nostoc, colonies arise, and the 
colony may propagate vegetatively as if it were an individual by 
dividing into groups of individuals or hormogonia. If the offspring 
produced by fission remain not only attached but in intimate or- 
ganic union, the result is not a colony of unicellular individuals but 
a multicellular individual composed of subindividual cells. What 
before was fission has become cell-division. Since the growth of 
multicellular organisms is entirely by cell-division, all growth may 
be traced back to the preparatory part of reproduction. 
We have seen that there is a great difference in the size of vege- 
table cells, some plants, such as Mucor, having the body consist of 
a single cell of relatively enormous size; and there are certain alge, 
less familiar, with a unicellular body very much larger than this. 
Such extraordinary development of a single cell seems not to have 
