Notices respecting New Books. 311 



affection and sympathy felt for him by his friends, were well known 

 in his lifetime, and are now plainly shown by the voluminous, but 

 valuable, correspondence and the obituary notices preserved in this 

 Biography. 



The titles of 92 of Dr. CrolPs books and memoirs, from 1857 to 

 1890, are catalogued at pages 527-535. The following remarks 

 are applicable to some of them, more particularly of 1857, 1864- 

 75, and 1883-90. 



"His first work, entitled 'The Philosophy of Theism,' published 

 in 1851, at the age of thirty-six, endeavoured to define the relation 

 of Theism to the determination of molecular motion. He tried to 

 show that, for the production of any organism, two things are 

 necessary, — first, motion ; second, the determination of motion. 

 Mere vital force might account for motion, but the determination 

 of motion implies an idea, design, and a directing mind " (page 507). 



" To Dr. Croll belongs the rare merit of showing that, though 

 glacial cycles may not arise directly from cosmical causes, they may 

 do so indirectly. His first contribution to the subject was pub- 

 lished in 1864, but the development of his theory resulted in a 

 series of brilliant researches extending over a period of eleven 

 years, to 1875. He was led to investigate the problem of the 

 eccentricity of the earth's orbit and its physical relations to the 

 Glacial Period. By means of Leverrier's formulae, he calculated 

 tables of eccentricity for three million years in the past and one 

 million years in the future, with the view of determining the 

 periods of high eccentricity, which according to his theory were 

 coincident with cycles of extreme cold. He was further led to 

 consider the various physical agencies affecting climate, resulting 

 from periods of high eccentricity, of which by far the most impor- 

 tant is the deflexion of the ocean-currents " (see page 510). The 

 cause of these was a subject of much discussion ; and " these various 

 lines of research are intimately associated with the fundamental 

 question of the physical cause of climatic change." 



" Dr. Croll's investigations into the geological history of terres- 

 trial climate had led him to consider the question of the origin of 

 the sun's heat, and thence to reflect on the possible condition and 

 development of nebulae and stars. The latter chapters of the " 

 ' Discussions ' &c. above mentioned " were devoted to these sub- 

 jects, which he would fain have discussed more at length, had not 

 the increasing failure of his bodily powers warned him that, if he 

 wished still to return to that philosophy which was his first love, 

 he must husband his remaining strength. Nevertheless the attrac- 

 tion of these astronomic problems proved insuperable. He continued 

 to work at the subject, enlarging the scope of the investigation 

 until it embraced not the earth and the sun merely, but the origin 

 and development of the whole material universe. At last he fol- 

 lowed his usual method, — gathered together his various contribu- 

 tions to the subject, trimmed, enlarged, and modified them, and 

 published them in a separate volume, entitled ' Stellar Evolution 

 in its Relation to Geological Time.' The publication of that work 

 marks the close of his labours in more definite scientific inquiry. 



