HARDWICKE'S SCIENCE-GOSSIP. 



219 



fertilised should vary very little, while cross-fertili- 

 sation will occasionally produce oscillation so extreme 

 as to become the starting points of new specific 

 waves." A similar view was put forward by a writer 

 in the " Science Monthly" for April. My opinion, 

 if it is worth anything, is just the opposite, and I 

 gathered from the " Origin of Species " that Mr. Dar- 

 win was of the same mind. Shortly stated, I regard 

 cross-fertilization as a check to the law of variation. 

 Every individual is subject to the modifying influence 

 of its environments acting in antagonism to hereditary 

 forces. As a consequence no two individuals are 

 exactly alike. Every difference is either of advantage 

 or disadvantage to the species, and as the tendency 

 to vary, acting unchecked, would operate more 

 quickly than the rate of change in the environments, 

 it is exceedingly probable that the disadvantageous 

 points of difference would vastly out-number the 

 advantageous ones. Such a state of things would be 

 ruinous to tlie race, and it would be perpetuated by 

 self-fertilization, because this process would simply 

 exaggerate and make more positive the disadvan- 

 tageous character thus introduced. Cross-fertihzation, 

 however, steps in, and by this means the departures 

 from the normal type are reined in, as it were, by 

 the influence of other plants which do not manifest 

 the same eccentricity ; equilibrium is restored, an 

 average of character is maintained, and the fixity of 

 the species is secured as we find it. Nevertheless a 

 species must not be too rigid. To be successful it 

 must retain a certain amount of plasticity, because, 

 like the individuals mentioned above, no two sets of 

 environments are exactly the same. To allow for 

 this, self-fertilization is frequently permissible, and 

 indeed, we find special arrangements to secure it, 

 but naturalists generally admit that it plays its part 

 only in a limited degree or during limited periods in 

 the life history of the species. At any rate this 

 affords an opening for the slow rate of modification 

 necessary to keep pace with the slowly changing 

 conditions of its existence. Thus, this view accounts 

 for both the fixity of some and the variability of 

 other species. 



J. Hamson. 

 Bedford, 



Mr. Thomas Fletcher, of Warrington, has suc- 

 ceeded in making an elastic rubber tube perfectly gas- 

 tight and free from smell. The tubing just patented 

 by Mr. Fletcher is made of two layers of rubber, with 

 pure soft tin-foil vulcanized between. It is perfectly 

 and permanently gas-tight under any pressure, and 

 free from the slightest trace of smell after long-con- 

 tinued use, whilst it retains the flexibility and 

 elasticity of an ordinary rubber tube. The tube 

 has been in use for some time, and has been 

 thoroughly tested for months under continuous and 

 heavy pressures. 



MINERALOGICAL STUDIES IN THE ; 

 COUNTY OF DUBLIN. 



No. II. 



THE southern shore of Dublin Bay is remarkable 

 for a fine exposure of granite rock, extending 

 from the town of Blackrock, to a'point near the hill 

 of Killiney whence a beautiful coast view is obtained. 

 The granite is flanked on one side by a portion of 

 the upper carboniferous limestone formation which 

 stretches north and westwards, and on the other by a 

 narrow strip of mica schist. Beyond this, lies a series 

 of rocks of lower Silurian age, which is in turn 

 succeeded by a splendid section of the Cambrian 

 period, composing the headland of Bray. These are 

 all easily reached, and in fact can be well observed 

 in the course of a few hours' excursion. At the 

 inland side of Killiney rises a smaller eminence, 

 known as Rochestown Hill, where for some time a 

 granite quarry has been extensively worked. This 

 stone is of excellent quality, another quarry in the 

 same neighbourhood having afforded material for the 

 harbour and breakwater of Kingstown, well known 

 as an attractive watering-place, and packet station for 

 the mail steamers running to and from Holyhead, 



Starting from Kingstown one afternoon with speci- 

 men bag, hammer and chisel, a walk of less than half 

 an hpur brought me within sight of the Rochestown 

 quarry, which presented the usual appearance of such 

 places. There were cranes, a wooden house or two 

 for repairing tools, and appliances for weighing, 

 whilst a number of heavy crowbars were lying about. 

 The quarrymen had just ceased working, which 

 afforded a better opportunity for examining the rocks. 

 The granite appears on a close inspection to present 

 variations in quality, that is, there are portions which 

 may be considered coarse in texture, the constituent 

 minerals being all prominently defined. Large 

 foliated masses of mica are frequently found standing 

 out conspicuously. On the other hand, the quarry is 

 traversed here and there by veins of a much finer 

 quality, known to the geologist as eurite, in which 

 the mica is not nearly so evident. This latter variety 

 is remarkably clean-looking and attractive in appear- 

 ance, being highly crystalline, and appearing to 

 scintillate under artificial light. The stone of the 

 quarry generally gives good examples of the characters 

 both of felspar and mica. Taking up a specimen in 

 which felspar predominates, the eye is first attracted 

 by its distinct cleavage and pearly lustre. Viewed 

 obliquely, the cleavage plane presents a silky appear- 

 ance. When opaque quartz occurs with felspar 

 in the mass, they may often be mistaken for each 

 other by an inexperienced observer, particularly in a 

 dubious light, but cleavage being obtained, the 

 character of felspar is evident, as this feature is 

 entirely absent in quartz. The felspars have been 

 ascertained to be principally orthoclase, but albite 



L 2 



