^02 



NATURE 



\jfuly I, i8b'o 



and other forms that are not tropical, could have reached 

 their present habitat. The range of this Araucaria, 

 although greater by far than that of the other Eutactas, 

 is very definitely limited to a strip of coast in New South 

 Wales between the ISellingen, a small stream about 

 S. lat. 31' 40', and Cape York in Queensland, in nearly 

 10° S. lat. It does not approach, therefore, to within 

 nearly a thousand miles of the equator of heat, which is 

 several degrees north of the true equator. They must 

 either have crossed the equator from the south in pra;- 

 eocene times and subsequently become isolated and died 

 out m their northern habitat, or have been originally 

 indigenous to the north and retreated to their present 

 stations. A passage must have been made in either case, 

 for the present distribution of Coniferre is against the 

 supposition that any identical species could have extended 

 synchronously in lowlands in both hemispheres, widely 

 separated by the equator. If a general lowering of 

 temperature had favoured their passage, the pre-existing 

 tropical vegetation must have altogether died out, and 

 the existing equatorial vegetation would present a com- 

 paratively new aspect. The absence of any of the Coniferas 

 that have ever been met with fossil in the plains of the 

 tropical regions at the present day, and of any existing 

 strictly equatorial plants, such as Gneta among Conifers, 

 in the fossil floras, seems at first sight to show that it 

 does do so and therefore lends some colour to what is at 

 be.^t merely a very crude hypothesis. A simpler supposi- 

 tion than that of a general lowering of temperature in the 

 Tropics, until more facts are forthcoming, is that the 

 passage was effected across high land, some of which 

 may still remain in Sumatra and Java. 



The specific identity which is apparent, of this and 

 other Australian forms, with those of our Eocenes, proves 

 that some, at all events, of the at present purely Australian 

 genera, neitheroriginated norbecamedifterentiated,asBen- 

 tham supposed, in Australia. The endemic genera, he says, 

 never spread far out of it, the only exceptions appearing 

 in the iSIalay Archipelago, "especially Timor, New 

 Guinea, and Borneo, and a few as far as Southern 

 China."' Nothing could speak more eloquently of the 

 path the migrations have taken, than these remnants left 

 upon the road, nor go farther to prove the former connec- 

 tion with our antipodes, which the discovery in 1S14 by 

 Urown of 1 50 European plants, a number since greatly 

 increased, growing endemically in Austraha, first of all I 

 believe suggested to us. 



It may not be altogether a useless supposition to hazard, 

 that if, as Saporta supposes, plants originated mainly if 

 not wholly in northern regions, and migrated south, the 

 continents of the southern hemisphere may be actually 

 preserving, as in the present case, our Eocene flora, and 

 have been inhabited in Eocene times by the Jurassic 

 flora which preceded it, or by some intervening flora of 

 •which we have now but the scantiest records. 



From what has been said the Araucarias are seen to be 

 an archaic type, formerly most widely spread, now dying 

 out and only lingering in restricted areas in the southern 

 hemisphere, whose very specific differentiation was accom- 

 plished before the Eocenes began. May its value as food 

 and use as the chief timber tree in the districts it still 

 inhabits preserve it from an accelerated extermination at 

 the hands of man. J. Starkie Gardner 



ON SOME POINTS CONNECTED WITH 

 TERRESTRIAL MAGNETISM 

 'T'HE remarks in Nature, vol. xxii. p. 169, of Messrs. 

 -•■ De La Rue and Muller in connection with their most 

 interesting and important researches on rarefied gases 

 induce me to ask the privilege of stating somewhat more 

 fully than I did on June 17 what I conceive to be the 



' Eentham, " Flora Australiej sis," vol. vii. 



position filled by a working hypothesis such as that then 

 mentioned in the science of terrestrial magnetism. Let 

 me begin with the aurora. Here we have a phenomenon 

 which invariably accompanies magnetic storms, on which 

 occasions it occurs simultaneously over a large portion of 

 the globe. Again the recent researches of the above- 

 named gentlemen render it very probable that auroral dis- 

 plays do not occur at a very great height, while it is con- 

 ceivable that they may occur at times at an altitude of a 

 few thousand feet. Here then we have a phenomenon 

 which is intimately connected with sudden changes of the 

 earth's magnetism. To this we may add earth currents 

 as another phenomenon of the same kind, so that we have 

 earth currents and auroral displays invariably associated 

 with magnetic storms, when these are of marked violence. 

 Now what is the nature of this connection .' When we 

 examine the formal laws of these associated phenomena 

 we find that these lead us (almost irresistibly, as I think) 

 to conclude that earth currents and aurora: are secondary 

 discharges caused by sudden changes in the earth's mag- 

 netism, no matter how these changes are produced. So 

 strong is the evidence oi form in this instance that the 

 late eminent magnetician John Allan Broun expressed to 

 me his belief that earth currents and aurorje were 

 connected with magnetic storms in the way above 

 mentioned. 



If this be assumed as the most probable working hypo- 

 thesis, it is natural to take another step. If we have dis- 

 charges produced in stationary strata by a changing 

 magnet, may we not have discharges produced in moving 

 strata by a constant magnet, and may not the motions 

 and changes of motion produced by the sun in the upper 

 convection currents of the earth give rise to electric 

 phenomena which may explain the changes of terrestrial 

 magnetism .' Of course this is only a working hypothesis, 

 liefore it can possibly become an established theory we 

 must have obtained from Messrs. De la Rue and Miiller 

 and from other obsen'ers that full and complete infor- 

 mation regarding discharges in rarefied media which 

 they are rapidly affording [us, and we must likewise have 

 obtained fuller information than we now possess regard- 

 ing the directions and velocities of the convection currents 

 in the upper regions of the earth's atmosphere. When 

 this is done, the problem may be regarded as ripe for the 

 mathematical physicist who may proceed with his calcu- 

 lations and either dismiss the hypothesis as untenable 

 or increase the probability of its truth. 



But in the meantime we are not ripe for this, and all 

 that we can do is to regard the hypothesis as a working 

 one, and endeavour by its means to elicit new facts 

 regarding X\\tfonn of the diurnal and other variations of 

 terrestrial magnetism. I submit that in this respect the 

 hypothesis has not been devoid of value. I have by its 

 means been led to derive the fact that certain magnetic 

 diurnal changes lag behind corresponding solar changes, 

 just as meteorological changes would do —a fact which 

 has since been confirmed by Air. Ellis of the Greenwich 

 Observatory. And I may be allowed to anticipate the 

 results of work at which I am now engaged so far as to 

 say that in the short periods which I am now investigat- 

 ing an increase or decrease of solar activity corresponds 

 to an increase or decrease both of magnetical and 

 meteorological activity. 



Again, in conjunction with others, I have shown by 

 preliminary discussions the probability of a progress of 

 magnetic phenomena from west to east just as we know 

 there is a progress of meteorological phenomena, only 

 magnetic weather (if I may use the expression) appears 

 to travel faster than meteorological weather. This last 

 appears to me to furnish almost a crucial test in favour of 

 this hypothesis, and through the courtesy of the Kew 

 Committee, the Astronomer Royal, and Mr. Carpmael of 

 Toronto I hope to be able soon to investigate this pheno- 

 menon in a more complete manner. 



