518 



THE GEOLOGIST. 



According to these views, every vertebrate skeleton is divided into 

 a number of transverse segments, perpendicular to the general direction 

 of the vertebral column or back-bone, each segment of the neck, the 

 back, and the tail, comprising a single vertebra, and each segment of 

 the head comprising a bone analagous to a vertebra. Immediately 

 above the central bone of each vertebra (the spinal column being sup- 

 posed horizontal) is the spinal canal, extending from the head to the 

 tail, and containing the spinal-chord, which is a continuation of 

 the mass, constituting the brain, the cavity of the skull being re- 

 garded as no expansion of the spinal canal ; and so with the limbs, and 

 throughout the entire organized frame, each segment of the body, 

 however modified, being regarded as the homological equivalent of each 

 and every of the others. 



The opposition of the early transcendentalists to the doctrine of 

 final causes — as the argument of Cuvier, of the adaptation of the organs 

 to the fulfilment of pre-ordained and special purposes is called — has con- 

 tributed to create a suspicion in the minds of many persons that all 

 such generalizations of the phenomena of organic life were inimical to 

 the latter doctrine, and tend to the notion that the adaptions and har- 

 monies of the adaptive modifications are in subservience to the general 

 laws. This mode of expression ought rather to be inverted, and it should 

 be asserted that these laws were impressed upon matter in subservience 

 to the ends to be obtained. Mr. Hopkins hence goes on to specula- 

 tions on the future conditions of our planets. The character of a 

 geologist," he says, " is essentially that of a historian; but it is im- 

 possible for him, at times, not to assume something also of a prophetic 

 character, and endeavour to stretch his mental gaze into the far future, 

 as he habitually directs it to the remote periods of the past. In ven- 

 turing such speculations in regard to the future of our globe, even those 

 extended chronological conceptions of time, which the contemplation of 

 geological phenomena has given, must be enlarged, and we must look 

 forward to times indefinitely more distant than those to which we may 

 have hitherto looked in our most remote anticipations of the future." 



As in the past history of the earth we have seen the important in- 

 fluence of heat, so, in turning to the future of our planet, we are led 

 first to inquire what may hereafter be expected to result from that uni- 

 versal agent in physical change ? and on this topic Mr. Hopkins con- 

 cludes " that the terrestrial heat is probably at presant, or must be 

 hereafter, reduced so as no longer to be of sufficient intensity to produce 

 a recurrence of those stupendous upheavals and dislocations which 

 have taken place in past times. 



The principal argument against the notion of any diminution of 

 intensity in the causes of such movements is based on tMfact that some 

 of the existing mountains— the Alps, for example — do not date from an 

 earlier period than the early Tertiaries. 



The next consideration is the future change of climatal conditions. 

 The mean annual temperature of the earth's surface depends on solar 

 and other external influences, the temperature of the surrounding pla- 

 netary space, and the constitution of the earth's atmosphere. 



It must, also, manifestly depend in part on the central heat, assuming 

 such to exist. The mathematical solution of the problem tells us that 



