UNSOLVED PROBLEMS OF SCIENCE. 43 



trine of the immutability of species. It has been mainly asso- 

 ciated in recent days with the honored name of Agassiz, but with 

 him has disappeared the last defender of it who could claim the 

 attention of the world. Few now are found to doubt that animals 

 separated by differences far exceeding those that distinguished 

 what we know as species have yet descended from common ances- 

 tors. But there is much less agreement as to the extent to which 

 this common descent can be assumed, or the process by which it 

 has come about. Darwin himself believed that all animals were 

 descended from " at most four or five progenitors " adding that 

 "there was grandeur in the view that life had been originally 

 breathed by the Creator into a few forms or one."' Some of his 

 more devoted followers, like Prof. Haeckel, were prepared to go a 

 step further and to contemplate a crystal as the probable ancestor 

 of the whole fauna and flora of this planet. 



To this extent the Darwinian theory has not effected the con- 

 quest of scientific opinion ; and still less is there any unanimity 

 in the acceptance of natural selection as the sole or even the main 

 agent of whatever modifications may have led up to the existing 

 forms of life. The deepest obscurity still hangs over the origin 

 of the infinite variety of life. Two of the strongest objections to 

 the Darwinian explanation appear still to retain all their force. 



I think Lord Kelvin was the first to point out that the amount 

 of time required by the advocates of the theory for working out 

 the process they had imagined could not be conceded without 

 assuming the existence of a totally different set of natural laws 

 from those with which we are acquainted. His view was not 

 only based on profound mechanical reasoning, but it was so plain 

 that any layman could comprehend it. Setting aside arguments 

 deduced from the resistance of the tides, which may be taken to 

 transcend the lay understanding, his argument from the refrigera- 

 tion of the earth requires little science to apprehend it. Every- 

 body knows that hot things cool, and that according to their sub- 

 stance they take more or less time in cooling. It is evident from 

 the increase of heat as we descend into the earth that the earth is 

 cooling, and we know by experiment, within certain wide limits, 

 the rate at which its substances, the matters of which it is con- 

 stituted, are found /o cool. It follows that we can approximately 

 calculate how hot it was so many million years ago. But if at 

 any time it was hotter at the surface by 50 Fahr. than it is now, 

 life would then have been impossible upon the planet, and there- 

 fore we can without much difficulty fix a date before which or- 

 ganic life on earth can not have existed. Basing himself on these 

 considerations. Lord Kelvin limited the period of organic life upon 

 the earth to a hundred million years, and Prof. Tait in a still 

 more penurious spirit cut that hundred down to ten. But on the 



