274 JouRNAL OF CoMPARATIVE NEUROLOGY. 
spects. This might rudely be schematized by saying that if you 
were to take all the cells of the human body and spread them 
over surface of the ocean they would have to spread over an 
expanse which would contain as much nourishment as is found 
in the fluids of the body. 
Now wherever the plant or animal passes from the water 
to the land environment it must carry its fluid medium with it. 
The most important distinction between the unicellular and the 
multicellular forms is that the multicellular form in this way 
controls the medium which surrounds its cells, ‘while the uni-- 
cellular form moves about in a medium over which it has no 
control, simply picking up what food it can. The multicellular 
form keeps its interior fluids at an even temperature by protec- 
tive coverings and a vasomotor mechanism, and restores the 
nutritive fluids regularly by means of the digestion of food 
and the respiration of air.’ Animals obviously would follow 
plants in the passage to the land, since they are dependent 
upon them for food. Transition types are to be found today 
in the amphibians which live partly on land and partly in the 
water. Now as the plant puts on protective layers of cellulose 
to shield its growing cells from destructive changes in the envir- 
oning conditions, it thereby presents a new problem to the ani- 
mal. These epidermal layers which protect the plant are an 
obstacle to the animal’s appropriation of it as food. The plant, 
so to speak, has erected a barrier between itself and the animal. 
Hence the terrestrial animal must develop in corresponding 
complexity in order to overcome this barrier. How this ob- 
stacle is overcome we have seen as it is roughly indicated in the 
specialized mechanism for digestion in the ruminant.  Practi- 
cally the entire energy of the ruminant with its series of stom- 
achs (one of them a sort of bacteriological laboratory) is de- | 
voted to the breaking down of the cellulose tissue in which its 
nourishment lies. 
! Cf. the development of the amnion and the serolemna in the Amniotes 
when the proreptilia passed to a terrestrial life. The amphibia are devoid of 
these foetal membranes: they would have been superfluous in aquatic forms. 
