TRANSACTIONS OF THE SECTIONS. 73 



regions of Scandinavia. One shudders at the thought of the innumerable icebergs 

 with their loads of rock, which floated in the once deeper North Sea, and above the 

 hills of the three Ridings of Yorkshire, and lifted countless blocks of Silurian stone 

 from lower levels, to rest on the precipitous limestones round the sources of the 

 Kibble. 



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

 and are familiar 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 

 1000 ft. high, ice would not retain its solidity ; if so, it could not propagate a greater 

 pressure in any direction. This question of the excavating effect of glaciers is 

 distinctly a mechanical problem, reqmring a knowledge of certain data ; and till 

 these are supplied, calculations and conjectures are equally vain. 



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

 trine that the eartli contains in its burial-vaults, in chronological order, fonns 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 Palseotherian periods are familiar 

 to every geologist. What closed the career of the several races of plants and ani- 

 mals 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 thing, 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 prototj'pes 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 Tcrebratula of the Cotswold oolite, the dragonfly of our rivers from the 

 Lihelhda of the Lias, and even the Rhyndionellce and Lingulce of the modern sea 

 from the old species which swarm in the Palseozoic 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, accompanied 

 by special diversity in every case (" descent with modification "), is recognized 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 are adaptable to new conditions ; if 

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

 animal may siu'vive in presence of slowly altered circumstances, but must perish 



