126 John Gunn — Changes of Climate. 



in the same direction. That of the orbit of Jupiter performs its 

 revolution round the sun in 186,207 years, while the perihelion of 

 Mercury's orbit occupies more than 200,000 years in completing its 

 circuit round the sun. To effect a complete restoration of the 

 planetary orbits to their original position with reference to their 

 perihelion-points will require a grand compound cycle amounting to 

 millions of years. Yet the time will come when all the oi'bits will 

 come again to their primitive positions, to start once more on their 

 ceaseless journeys." 



Of these three great astronomical cycles, we have to deal, geolo- 

 gically, chiefly with the first, the Precessional ; the second, the 

 Perihelionic, affects us mainly by shortening the Precessional orbit ; 

 the third is supposed to act in extremely long periods of time, 

 millions of years, which are invoked by a very high authority, to 

 account for the Great Ice Age. 



I propose first to consider the effects produced upon the Fauna and 

 Flora by Precession, which are independent of the elevation of 

 mountain-ranges. 



Herbert Spencer (Essays, 1863, Second Series, p. 87) gives an, 

 admirable and precise detail of the phenomena attendant upon the 

 Precessional cycle. 



"It seems," he observes, "beyond all question that there must 

 have been a consequent rhythmical change in the distribution of 

 organisms, a rhythmical change to which we here wish to draw 

 attention, as one cause of minor breaks in the succession of fossil 

 remains. Each species of plant and animal has certain limits of heat 

 and cold within which only it can exist, and these limits in a great 

 degree determine its geographical position. It will not spread north 

 of a certain latitude, because it cannot bear a more northern winter, 

 nor south of a certain latitude, because the summer heat is too great; 

 or else it is indirectly restrained from spreading further by the effect 

 of temperature, on the humidity of the air, or on the distribution of 

 the organisms it lives upon. But now, what will i-esult from a slow 

 alteration of climate produced as above described ? Supposing the 

 period we set out from is that in which the contrast of seasons is 

 least marked, it is manifest that during the progress towards the 

 period of the most violent contrast, each species of plant and animal 

 will gradually change its limits of distribution, will be driven back, 

 here by the winter's increasing cold, and there by the summer's 

 increasing heat — will retire into those localities which are still fit 

 for it. Thus during 10,000 years each species will ebb away from 

 certain regions it was inhabiting, and during the succeeding 10,000 

 will flow iDack into those regions. From the strata there forming, 

 its remains will disappear ; they will be absent from some of the 

 superimposed strata, and will be found in strata higher up. But in 

 what shapes will they reappear? Exposed during the 2(1,000 years 

 of their slow recession and their slow return, to changing conditions 

 of life, they are likely to have undergone modifications ; and will 

 probably reappear with slight differences of constitution, and perhaps 

 of form — will be new varieties, and perhaps new species." 



