NA TURE 



{June 14, 1877 



all his spare time was devoted to the committing of 

 Euclid to memory ! I shuddered as I thought of what 

 was to be my own fate in a few short months, when I too 

 must be subjected to this fearful imposition. But the first 

 hour or two which Dr. Gloag (a name strange, perhaps, to 

 southern ears, but very high indeed on the roll of success- 

 ful teachers — Clerk-Maxwell, indeed, was one of his pupils) 

 devoted to geometry showed those of us who had any taste 

 for the subject that it was one to be learned by head, not 

 " by heart" (the idiotic phrase in common usej — and that 

 my friend's parents had simply taken him from a good 

 teacher and sent him to an exceedingly bad one — for it 

 came to be discovered after some time that he had really 

 considerable aptitude for geometry. 



But if he had been in fact quite unfit for the study, 

 otherwise than in learning to repeat Euclid by rote, what 

 object beyond mere torture would have been attained by 

 forcing it upon him ? This leads to another remark of 

 great importance in connection with the mass of 

 elementary te.xt-books. 



What sort of students are those who require to be told 

 to take the square of the velocity, divide it by the radiiist 

 and find the propoi'tion of t/iis quotient /o 32 . . . : — 

 without farther explanation or proof ? What the better 

 are they of the information ? Call you this " teaching 

 science?" Has it improved their minds? Will they be 

 able to make any use of it in after life ? I do not see 

 how these questions and many other connected ones can 

 be answered except by a prompt negative. One of two 

 things. The pupil who requires to be taught in this way 

 is either as yet too young, or is one who will never be- 

 come old enough, to learn even the rudiments of science. 



To our metaphor once more. Grass-plats, moss, and 

 flower-beds for the happy sports of children — the bare 

 rock and rough moor for the stern work of men. Your 

 gravel-walks and Macadamised roads are excellent things 

 in their way, but keep them to their legitimate users, the 

 carriage and the perambulator for the invalid and the 

 infant who can neither work nor even play. 



My reasons for writing on this subject are very serious 

 ones. I have to consider each year howbest to instruct some 

 couple of hundred students in the elements of physics, and 

 have to be constantly on the out-look for a really good 

 text-book of an elementary character. In the higher 

 branches of the subject there is, happily, little difficulty, 

 but that a really good, short, and simple treatise on the 

 merest elements has been (at least till very lately) wholly 

 unprovided is, I think, clear from the ridiculous discus- 

 sions about Cenlrifui;al Force, and other connected ideas, 

 which are even now constantly to be found in our more 

 practical periodicals. P. G. Tait 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neitlier can he underlain to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No 7totice is taken of anonymous communications. 



The Editor urgently requests correspondents to keep their letters as 

 short as posiible. The presitire on his space is so great that it 

 is impossible otherwise to ensure the appearance aen of com- 

 munications containing interesting and novel facts.} 



Nectar-Secreting Glands 



Mr. Francis Darwin has made an interesting addition to 

 his important discovery of uectar-bearing glands on the young 



fronds of Pteris aquilitia, supplied from the ever-welcome expe- 

 rience of Mr. Fritz MuUer. The latter gentleman finds that in 

 Brazil the Pteris aquilina is protected from the leaf-cutting ants 

 by those attracted to the nectar, and Mr. Darwin sdds some 

 speculations on the origin of the glands and their continued 

 functional activity in Europe where they now appear to be use- 

 less. On this i)art of the question I should like to make the 

 following remarks : — 



Prof. Ileer has shown that in the Miocene plant-beds at 

 (P.ningen and Radoboj, ants are the most numerous amongst the 

 fossil insects, and in 1S49 as many as sixty-six species had been 

 described from these two localities. In 1S65 the number found 

 at GIningen alone is recorded as forty-four. I do not know 

 what the total number of species is th.it have been recorded from 

 the two places up to the present time, but it prob.ibly does not 

 fall short of eighty. Amongst the fossil ants from Radoboj there 

 are species of the Tropical American genera Atta and Ponera. 

 One of the fossil species of Alia resembles in general form and 

 in the venation of the wings the curious Alia cephalotes of Tropical 

 America. 



As there are only about forty species of ants existing now in 

 the whole of Europe it is evident that in the Miocene epoch they 

 must have played a much more important part in Europe than 

 they do now. Plants may then have been exposed to the attacks 

 of enemies that have become extinct along with the general im- 

 poverishment of the fauna and flora of Europe that took place in 

 Post-pliocene times ; and the protection afforded by ants attracted 

 to the nectar-bearing glands at the critical stage of the unfolding 

 of the young and tender leaves may have been as important to 

 some plants in Europe, then, as it is to many in Tropical America 

 now. 



With regard to the persistency of the nectar-producing glands 

 up to the present time in Europe, it is to be remarked that many 

 plants are identical with those living in the Miocene period and 

 the world-wide distribution of Picris aquilina seems to indicate 

 thatit isof very ancient origin. If a plant has not otherwise varied 

 there is no reason apparent why it should do so in this respect so 

 long as the secretion of nectar is not positively injurious to it. I 

 have recently noticed in ray garden that the ants that attend the 

 glands at the bases of the leaves of the cherry, the plum, the 

 peach, and the apricot, stroke with their antenna; some of the 

 glands that are not excreting when they arrive at them, just as 

 they do the bodies of the aphides. I have not actually noticed 

 that this promotes a flow of nectar, but ever since I became a 

 disciple of Darwin I have been convinced that the most 

 trivial circumstance is worthy of notice ; and it may be that 

 the slight irritation of the glands kept up by the ants is suffi- 

 cient to ensure the perpetuation of a function of the plant now 

 useless to itself. It is, however, perhaps too soon to assume 

 that the glands are entirely useless to the plants in Europe. 

 Darwin states that there is good evidence that the absence of 

 glands in the leaves of peaches, nectarines, and apricots leads to 

 mildew ("Animals and Plants under Domestication," vol. ii. 

 P- 231). 



Darwin refers at the same place to the variation of the glands 

 of the leaves in the above-mentioned fruit trees and I may add 

 that they are extremely variable on the cherry, being sometimes 

 absent, sometimes on the stalk and sometimes on the blade of 

 the leaf. The young leaf in its earliest stage, before it expands, 

 has a complete fringe of them, thus bearing out Mr. Francis 

 Darwin's theory that they are homologous with the serration- 

 glands of Reinke. 



May I suggest to some of your correspondents that information 

 as to how far north in Great liritain or in Europe the glands on 

 the above fruit trees are attended by ants and especially if the 

 wild cherry (which I have not had an opportunity of observing) 

 is so attended, would be of great interest. Thomas Belt 



Cornwall House, Ealing, June S 



On Time 



"The fact is, that we have not yet quite cast ofi" the tendency to iO-called 

 metaphysics." — Tait, ''Rec. Adv. in Phys. Sc," p. ji. 



In Thomson and Tait's "Natural Philosophy," of which I 

 have only the German edition in my possession, I find, § 246 : 

 " Die Zeiten, wiihrend welcher irgend ein besonderer Korper, tier 

 dutch keine Kraft angetrieben wird, die Geschwindigkeit seiner 

 Bewegung zu iindern, gleiche Wege durchlauft, sind einander 

 gleich." And § 247 : " Dieser Satz druckt bloss die fur die 

 Messung der Zeit allgemein getroffene Uebereinkunft aus." 



These quotations quite express what is generally understood, 



