3 2 4 



NATURE 



[September 7, 191 1 



chemical analyses ol the chiei rock-types, weighted accord- 

 ing to their relative abundance, it is possible to calculate 

 approximately the composition of the parent-magma oi a 

 province. Noting that nearly identical assemblages of 

 rocks sometimes occur in widely separate provinces and at 

 different geological periods, we have some reason foi 

 expecting that the provincial parent-magma.s ma) ulti- 

 mately be reduced to a limited number of types. Whether 

 these types $t\\\ be suffii I di finite to serve as a ba i i 

 ation it is too early to say. 



For the sake of argument, I have taken chemical com- 

 position as the criterion. It is certain, however, that a 

 rock-magma consist-, not oi free oxides, but mainly of 

 silicate-compounds, and the variation produced by mag- 

 matic differentiation is a variation in the relative propor- 

 tions of such compounds. The characteristics common to 

 a set of cognate rock-types will therefore be more properly 

 expressed in mincralogical than in chemical terms. If, to 

 fix ideas, we take as representative of a province its prin- 

 cipal plutonic series, we shall often find that some par- 

 ticular mineral or some special association of minerals 

 stands out as a distinctive feature. For instance, in the 

 charnockite-norite series of southern India the character- 

 istic ferro-magnesian mineral is hypersthene ; in the 

 granite-gabbro series of the British Tertiary it is augite ; 

 and in the granite-diorite series, which predominates among 

 the " newer granites " of the Scottish Highlands, horn- 

 blende and biotite. These three sets of rocks, all of calcic 

 facies, are easily distinguishable in isolated specimens. 



Each such rock-series embraces types ranging from acid 

 to ultrabasic. This variation is ascribed to a later 

 differentiation of the parent-magma of the province, and, 

 therefore, in an arrangement based on genetic principles, 

 it will find expression, not in the main divisions of the 

 scheme, but in the subdivisions. Here is an essential 

 difference between an ideal petrogenetic classification and 

 the petrographical systems which are, or have been, in use. 

 If we are content to limit our study of igneous rocks to 

 specimens in a museum, the distinction of acid, neutral, 

 basic, and ultrabasic may seem to be one of first import- 

 ance. It has, in fact, been employed for the primary 

 divisions in some formal schemes, e.g. in that put forward 

 by Lowinson-Lessing. In a less crude system, like that 

 of Rosenbusch, this element disappears, but the underlying 

 idea still remains. There is a division into families, such 

 as the granite-family and the gabbro-family, but the term, 

 in so far as it implies blood-relationship, is. a misnomer. 

 The augite-granite of Mull is evidently more closely related 

 to its associated gabbro than it is, say, to the biotite- 

 granite of Peterhead or the hypersthene-granite of Madras. 

 The differentiation which evolves a varied series of 

 plutonic rocks from a common parent-magma is clearly 

 not of the same kind as that which gave rise to the 

 parent-magma itself. It appears that the external 

 mechanical element is here a less important factor, and 

 the variation set up is, therefore, more closely in accord- 

 ance with the uninterrupted course of crystallisation. This 

 is clearly indicated when we compare the order of intrusion 

 of the several rocks of the series with the order of 

 crystallisation of their constituent minerals. The history 

 of th'- series is in a sense epitomised in the history of each 

 individual type, corresponding in both cases with continued 

 fall of temperature and progressive change in the composi- 

 tion of the residual magma. In a large number of rocks, 

 more particularly those of complex constitution, the order 

 of crystallisation follows Rosenbuseh's empirical law of 

 decreasing basicitv. and the plutonic intrusions then begin 

 with the most basic type and end with the most acid. I 

 mention this only to point out that, while the larger 

 divisions of our ideal classification will have a certain 

 geographical and tectonic significance, the subdivisions will 

 show a certain correspondence with the sequence in time 

 various cognate rock-1 

 To pursue the subject farther would serve no useful 

 purpose. It is clear that, if a natural — by which I mean 

 a genetic — classification of igneous rocks is ever to become 

 a reality, much work must first be done, both in the field 

 and in the laboratory, each petrographical province being 

 studied from the definite standpoint of the evolution of its 

 rork-tvpes from one parent stock. Such researche 



of Brogger in the Christiania province may serve as 



NO. 2184, VOL. 87] 



a model. It would be rash to venture at present more 

 than the most general forecast of the lines which future 

 developments may follow; but I think it calls for no 

 hardihood to set limits to what may ultimately be po 

 in this direction. There are those who would have us 

 abandon in despair all endeavour to place petrography 

 upon a genetic basis, and fall back upon a rigid arbitrary 

 11 as a final solution of the difficulty. This would be 

 to renounce for ever the claim of this branch of geology 

 to rank as a rational science. I have said enough to 

 show that I am one of those who take a more hopeful 

 view of the future of petrology, confidently expecting it to 

 show, like the past, a record of continued progress. 



SECTION D. 



ZOOLOGY. 



i\g Address by Prof. D'Arcv Wentworth 

 Thompson, C.B., President of the Section. 



Magnolia Naturae; or. The Greater Problems of Bioi 



The science of Zoology, all the more the incorporate 

 science of Biology, is no simple affair, and from its 

 earliest beginnings it has been a great and complex and 

 many-sided thing. We can scarce get a broader view of it 

 than from Aristotle, for no man has ever looked upon our 

 science with a more far-seeing and comprehending eye. 

 Aristotle was all things that we mean by " naturalist " or 

 "biologist." He was a student of the ways and doings 

 of beast and bird and creeping thing; he was morphologist 

 and embryologist ; he had the keenest insight into physio- 

 logical problems, though lacking that knowledge of the 

 physical sciences without which physiology can go but a 

 little way : he was the first and is the greatest of psycho- 

 logists ; and in the light of his genius biology merged in 

 a great philosophy. 



I do not for a moment suppose that the vast multitude 

 of facts which Aristotle records were all, or even mostly, 

 the fruit of his own immediate and independent observa- 

 tion. Before him were the Hippocratic and other schools 

 of physicians and anatomists. Before him there were 

 nameless and forgotten Fabres, Roesels, Reaumurs, and 

 Hubers, who observed the habits, the diet, and the habita- 

 tions of the sand-wasp or the mason-bee ; who traced out 

 the little lives, and discerned the vocal organs, of grass- 

 hopper and cicada ; and who, together with generations of 

 bee-keeping peasants, gathered up the lore and wisdom of 

 the bee. There were fishermen skilled in all the cunning 

 of their craft, who discussed the wanderings of tunny and 

 mackerel, sword-fish or anchovy ; who argued over the 

 ages, the breeding-places, and the food of this fish or 

 that; who knew how the smooth dogfish breeds two 

 thousand years before Johannes Miiller ; who saw how the 

 male pipe-fish carries its young before Cavolini ; and who 

 had found the nest of the nest-building rock-fishes before 

 Gerbe rediscovered it almost in our own day. There were 

 curious students of the cuttle-fish (I sometimes imagine 

 they may have been priests of that sea-born goddess to 

 whom the creatures were sacred) who had diagnosed the 

 species, recorded the habits, and dissected the anatomy of 

 the group, even to the discovery of that strange hecto- 

 cotylus arm that baffled Delia Chiaje, Cuvier, and 

 Koelliker, and that Verany and Heinrich Miiller re- 

 explained. 



All this varied learning Aristotle gathered up and 

 into his great web. But every here and there, in words 

 that are unmistakably the master's own, we hear him 



of what are still the great problems and evei 

 hidden mysteries of our science: of such things as the 

 nature of variation, of the struggle for existence, of specific 

 and generic differentiation of form, of the origin of the 

 tissue-;, the problems of heredity, the mystery °f sex. of 

 the phenomena of reproduction and growth, the character- 

 istics of habit, instinct, and intelligence, and of the very 

 meaning of Life itself. Amid all the maze of concrete 

 facts that century after century keeps adding to our store, 

 these, and such as these, remain the ■ ol 



natural science— the Magnolia naturae, to borrow a great 

 word from B'.con, who in his turn had borrowed it from 

 St. Paul. 



