476 Reports and Proceedings — 



Those who, with Professor Ramsay, adopt the glacial hypothesis in its full 

 extent, and are famihar with the descent of ice in Alpine valleys where it grinds 

 and polishes the hardest rocks and winds like a slow river round projecting cliffs, 

 are easily conducted to the further thought that such valleys have been excavated 

 by such ice-rubbers, and that even great lakes on the course of the rivers have been 

 dug out by ancient glaciers which once extended far beyond their actual limits. 

 That they did so extend is in several instances well ascertained and proved ; that 

 they did in the manner suggested plough out the valleys and lakes- is a proposition 

 which cannot be accepted until we possess more knowledge than has yet been 

 attained regarding the resistance offered by ice to a crushing force, its tensile 

 strength, the measure of its resistance to shearing, and other data required for a 

 just estimate of the problem. At present it would appear that under a column of 

 its own substance looo ft. high, ice would not retain its solidity ; if so, it could not 

 prcjpagate a greater pressure in any direction. This question of the excavating 

 effect of glaciers is distinctly a mechanical problem, requiring a knowledge of 

 certain data ; and till these are supplied, calculations and conjectures are equally 

 vain. 



A distinguishing feature of modern geology is the great development of the doc- 

 trine that the earth contains in its burial-vaults, in chronological order, forms of 

 life characteristic of the several successive periods when stratified rocks were depo- 

 sited in the sea. This idea has been so thoroughly worked upon in all countries, 

 that we are warranted to believe in something like one universal order of appear- 

 ance in time, not only of large groups but even of many genera and species. The 

 Trilobitic ages, the Ammonitic, Megalosaurian, and Paljeotherian periods are 

 famihar to every geologist. What closed the career of the several races of plants 

 and animals on the land and in the sea, is a question easily answered for particular 

 parts of the earth's surface by Reference to "physical change";: for this is a main 

 cause of the presence or absence, and in general of the unequal distribution of life. 

 But what brought the succession of different races in something like a constant 

 order, not in one tract only, but one may say generally in oceanic areas, over a 

 large portion of the globe ? 



Life unfolds itself in every living things from an obscure, often undistinguishable 

 cell germ, in which resides a potential of both physical and organic change — a 

 change which, whether continual or interrupted, gradual or critical, culminates in 

 the production of similar germs, capable' under favourable conditions of assuming 

 the energy of life. 



How true to their prototypes are all the forms with which we are familiar, how 

 correctly they follow the family pattern for centuries, and even thousands of years, 

 is known to all students of ancient art and explorers of ancient catacombs. But 

 much more than this is known. Very small differences separate the elephant of 

 India from the mammoth of Yorkshire, the Waldheimia of the Australian shore 

 from the Terebratula of the Cotswold Oolite, the dragon-fly of our rivers from the 

 Libellula of the Lias, and even the Rhyiichonellce and Lingulce of the modern sea 

 from the old species which swarm in the Palaeozoic rocks. 



But concurrently with this apparent perpetuity of similar forms and ways of life, 

 another general idea comes into notice. No two plants are more than alike ; no 

 two men have more than the family resemblance \ the offspring is not in all respects 

 an exact copy of the parent. A general reference to some earlier type, accom- 

 panied by special diversity in every case ("descent with modification"), is re- 

 cogniised in the case of every living being. 



Similitude, not identity, is the effect of natural agencies in the continuation of 

 life-forms, the small differences from identity being due to limited physical con- 

 ditions, in harmony with the general law that organic structures are adapted to the 

 exigencies of being. Moreover the structures ai^e adaptable to new conditions ; if 

 the conditions change, the structures change also, but not suddenly ; the plant or 

 animal may survive in presence of slowly altered circumstances, but must perish 

 under critical inversions. These adaptations, so necessary to the preservation of a 

 race, are they restricted within narrow limits? or is it possible that in the course of 

 long-enduring time, step by step and grain by grain, one form of life can be 

 changed and has been changed to another, and adapted to fulfil quite different 

 functions ? Is it thus that the innumerable forms of plants and animals have beeix 

 "developed" in the course of ages upon ages from a few original types? 



