1908-9. | ON THE ORIGIN OF LIFE ON THE GLOBE. 439 
As to the temperature preceeding this period of development, 
Chamberlin is not quite certain, for his language is as follows :—* 
“There was, however, a terrestrial source of heat and light of 
critical importance, namely, that arising from the infall of the plane- 
tesimals. If this infall were at a rate sufficient to heat the surface of 
the earth above 100° C, life of the present types would have been pro- 
hibited. The present stage of the inquiry does not permit any very 
confident opinion as to whether this excess was reached or not. Leaving 
this question open, it is to be noted that if, at the stage when an atmo- . 
sphere and hydrosphere could be held, the infall of planetesimals was so 
rapid as to heat the surface to a prohibitive temperature, the rate of infall 
must almost certainly have declined as the number of planetesimals in 
the earth’s feeding zone was diminished so that, long before the supply 
was exhausted and growth ceased, the rate must inevitably have fallen 
below the prohibitive limit. If, therefore, the earth were too hot for life 
when one-fifth grown, its temperature might have become suitably mild 
when one-fourth, one-third, one-half, or three-fourths grown. Growth 
after this permissive stage was reached would be slow and the period 
required for its completion would still be long.” 
it is obvious therefore, that even the Planetesimal Theory does not 
negative the view that high temperatures did obtain in the earth’s 
atmosphere before a hydrosphere was formed or even after one was 
developed, and that the conditions which would obtain, on the basis of 
the Nebular Theory, as favorable to the origin of life, would obtain in 
the planetesimal development of the earth. 
Just at what period in the earth’s history these conditions obtained 
it is not now possible to say. We know that highly developed forms 
like Olenellus, forms quite as highly developed and specialized as 
vertebrates. appeared in the early Cambrian Period, and therefore, the 
development of such highly specialized forms must have begun at a 
vastly earlier stage. But back of all this is a period incalculably longer, 
a period during which the cell was specializing into animal and vege- 
table types and back still of that period was another of still greater 
duration which was required to develop in the cell, still neither animal 
nor vegetable definitely, those peculiar processes which the animal as 
well as the vegetable cell now manifests when it undergoes what is 
known as cell division. 
To have fixed in the cell, animal and vegetable, those mitotic 
* Carnegie Institution Year Book, No. 3, 1905, p. 250. 
