go8 LIFE : OUTLINES OF GENERAL BIOLOGY 



increasing rate it will be long before our museums are as complete 

 as may be, and longer still before the questions they arouse are 

 adequately answered. 



The greater discoveries in European palaeontology were mainly 

 before the epoch-making Darwinian movement, and though dis- 

 tinguished work has of course continued to be done, and by many 

 investigators, of whom we may take Kowalewsky as an outstanding 

 example, the most striking harvests of new forms, mammalian and 

 reptilian especially, have been those rewarding North American 

 workers after that date, as so notably in the last generation: 

 witness Cope, Marsh, Leidy, etc., and now more recently Osborn, 

 Lull, and others. From Lull's masterly outline of Organic Evolution 

 we may here cite a passage, from his epilogue, entitled "The Pulse 

 of Life", since asking one or two of such main questions as are above 

 indicated, and with broadly outlined answers; and these also of 

 interest as characteristic of the large way in which palaeontologists 

 are nowadays thinking: 



"The stream of life flows so slowly that the imagination fails to 

 grasp the immensity of time required for its passage, but like many 

 another stream it pulses irregularly as it flows. There are times of 

 quickening, the expression points of evolution, which are almost 

 invariably coincident with some great geological change, and the 

 correspondence is so exact and so frequent that the laws of chance 

 must not be invoked by way of explanation. The geologic changes 

 and the pulse of life stand to each other in the relation of cause 

 and effect. This statement does not, however, imply the acceptance 

 of the Lamarckian factor any more than that of natural selection, 

 for whether the influence of a changing environment acts directly 

 upon the creature's body, or indirectly through induced habit, or 

 whether it merely sets a standard to which animals must conform 

 if they would survive, matters not; the fundamental principle 

 remains that changing environmental conditions stimulate the 

 sluggish evolution stream to quickened movement" (LuU's Organic 

 Evolution, 1917, p. 687). 



This is well said, and the author has been notably successful in 

 correlating the pulse of life with influences of climate, chiefly 

 temperature and moisture variation, due to topographic or to general 

 atmospheric conditions. "Back of these climatic changes lies, as one 

 of the great fundamental causes, earth shrinkage, with a conse- 

 quent warping of the crust which produces mountain ranges and 

 enlarges the lands. Thus it will be seen that the most momentous 

 changes, so far as influence on life is concerned, may have, geologi- 

 cally speaking, a very simple basic cause." Yet in thinking of the 

 external causes that influence the course of evolution, we must not 

 forget that we started with "the stream of life". 



