272 



NATURE 



[Jan. 23, 1890 



I am also quite willing to admit that there may be other 

 causes tending to raise the zero-point besides the equalization of 

 tension, such, for instance, as the chemical changes alluded to 

 by Prof. Mills ; but I should like to ask, as I am ignorant on the 

 point, whether there is any experimental evidence of their 

 nature or existence. Sydney Young. 



University College, Bristol, January 11. 



Foreign Substances attached to Crabs. 



In your issue of December 26, and also in exhibiting his 

 collection of crabs before the Linnean Society, Mr. Pascoe cast 

 some doubt on the function of the two pairs of modified legs of 

 Dromia vulgaris, which are usually supposed to be adapted to 

 the retention of the sponge with which it covers its carapace. 



That these legs were really used for this purpose I was enabled 

 to observe, during my stay at the zoological station in Naples 

 last winter. I had in my tank several specimens, in some of 

 which the sponge had also extended on to the ventral surface, 

 over the edge of the carapace, thus securing a firm hold apart 

 from the action of the legs. In all specimens, however, there 

 are seen, when the sponge is removed, which requires con- 

 siderable force, two oblique depressions into which the legs fit, 

 giving them thus a distinct hold on the sponge. 



If the latter be, however, removed from the animal but left in 

 the tank, the crab soon sets to work to regain possession of its 

 covering, and can be seen to use its modified hinder pairs of 

 legs most effectually for this purpose. It would seem therefore 

 beyond doubt that these modified legs serve not only for holding 

 oh the sponge, but also for getting hold of a new sponge, should 

 the old one get injured or die, as must happen not unfrequently. 



F. Ernest Weiss. 



The Zoological Laboratory, University College, January 6. 



Galls. 



I AM sorry if I unintentionally misrepresented the opinions of 

 Prof. Romanes and Dr. St. George Mivart in suggesting that 

 they wished to assail the theory of natural selection in their 

 recent communications to Nature on this subject. They must, 

 however, pardon me for saying that I still think the extract 

 to which I alluded in my note admits this interpretation. As 

 my views of the relations of gall-formation to the theory of 

 natural selection are clearly at variance with those of your corre- 

 spondents, perhaps you will allow me space to give briefly the 

 grounds upon which I base my conclusions. 



There are in England about ninety well-known varieties of 

 galls, and of this number fully a third are found in the oak. 

 About half the oak-galls are formed on growing leaves. In 

 nearly one-third of the total number the grub is hatched, and 

 the gall is fashioned in a developing bud. We can readily 

 imagine, in the case of a tree with deciduous leaves, that the 

 presence of a few galls upon its foliage would not greatly affect 

 its chances of survival, if its fitness was in other respects com- 

 plete. It is otherwise when a gall occupies the position of a 

 developing bud, especially when the bud is a terminal one. In 

 this case there occurs coincidently with, and as a result probably 

 of, the adventitious formation, an arrest of normal development 

 and growth. Indeed, I believe "the gnarled and twisted oak " 

 owes niany of its gnarls and most of the twists to the common 

 oak-apple and other bud-galls. If a tree endowed with less 

 developmental vigour and with fewer supplementary buds than 

 the oak had been exposed to the repeated attacks of the insects 

 for many generations in a struggle for existence, it would doubt- 

 less have long ago succumbed, and it would have done so by a 

 process of natural selection operating in the ordinary manner, 

 and not " at the end of a long lever of the wrong kind," what- 

 ever that may mean. This selective process in the case of gall- 

 bearing trees has left possible traces of its action to-day, for I 

 am unaware that any other English tree than the oak is attacked 

 by terminal bud-galls. The terminal leaf-galls of certain Salices 

 and Conifers can scarcely affect their growth and development 

 to the same extent as the bud-galls. 



When we compare pathological tumours in the higher animals 

 with these vegetable excrescences, we must make due allowances 

 for the different conditions under which each lives. I cannot 

 then see that the "morphological specialization" of galls, 

 which, for the most part, are composed of hypertrophied repro- 

 ductions of the simpler vegetable tissues, is greater than that 

 exhibited by man himself, when, for instance, he becomes the 



involuntary host of Dr. Lewis's Filaria;, and his leg the seat of 

 Elephantiasis lymphangiectodes, accompanied by hypertrophy 

 of many integumentary structures of the limb. Oak-spangles, 

 on the other hand, are to my mind comparable to the circular 

 nests of ringworm, or to the sprouting epithelium of a Verruca 

 necrogenetica. Such comparisons may be of little scientific 

 value, yet I take it they are as useful in their place as attempts 

 to gauge the amount of " disinterestedness" shown by a cabbage 

 when it becomes the unwilling host of the gall-producing 

 Ceuthorhynchus sulcicollis. W. Ainslie Hollis. 



Brighton, December 30, 1889. 



The Evolution of Sex. 



The interesting note of Mr. M. S. Pembrey in your issue of 

 January 2 (p. 199), induces me to draw the attention of your 

 correspondent to a short paper of mine just published (or in 

 course of publication) in the Ibis, where I communicated the 

 experiences of a friend, who had hatched a series of parrot 

 eggs, belonging to the genus Eclectus, in which the young 

 males are green, the young females rd. It is remarkable that 

 by far the larger number of the birds hatched were males. In 

 each case only two eggs were laid, and the breeder himself, with- 

 out being able to tell why, is of opinion that nearly all his 

 hatche- consisted of male birds. As there are still many embryos 

 of those Eclectus in my hands, the sex of which is not yet de- 

 termined, I hope to be able to make known the result of my in- 

 vestigation later, whether the pairs are always males, or always 

 females, or consist of a male and a female bird, at least sometimes. 

 Meanwhile, I should be glad to hear if anything more is known 

 about the sexes of birds which lay only two eggs at a time. 



A. B. Meyer. 



Royal Zoological Museum, Dresden, January 5. 



" Manures and their Uses." 



Allow me to thank the well-known writer "W." for his 

 review of the above-mentioned book. "W." does not hold 

 with the view that "farmyard manure is erroneously supposed 

 to contain all the necessary plant-foods required for the growth 

 of plants." I believe, with M. Ville and others, that "the 

 farmer who uses nothing but farmyard manure exhausts his 

 land." " W." speaks of this as an "obvious fallacy." If the 

 statement is wrong, would "W. " kindly answer the quotation 

 given on p. 76 of the book in question. The quotation "runs " 

 as follows : — 



"M. Grandeau (the French agricultural authority) recently 

 estimated that one year's crop in France represents 298,200 

 tons of phosphoric acid, of which only 151,200 tons were re- 

 covered from the stable dung, thus leaving a deficit of 147,000 

 tons, equal to over one million tons of superphosphate, to be 

 made good by other means. 



"M. Grandeau also estimated that the entire number of farm 

 animals in France in 1882, representina^ a live weight of 

 6,240,430 tons, had accumulated from their food 193,453 tons of 

 mineral matter containing 76,820 tons of phosphoric acid. 

 These figures give some idea of the enormous quantities of phos- 

 phoric acid required to restore to the soil what is continually 

 bemg carried away by the crops sold off the farm." 



It must be borne in mind that in the above estimates, M. 

 Grandeau includes the purchase of oil-cakes and other feeding 

 stuffs. Therefore, if farmyard manure only contains about half 

 the amount of phosphoric acid (to say nothing of nitrogen, 

 potash, &c. ) required to retain the land in a fertile condition, 

 how can I have attached "too much prominence to chemical 

 manures, and too little importance to stock-feeding as a manurial 

 agency"? A. B. Griffiths. 



[Dr. Griffiths assumes that because, as asserted by M. 

 Grandeau, the balance of fertilizing matter in France is against 

 the land, "the farmer who uses nothing but farmyard manure 

 exhausts his land. " This is arguing from general principles to 

 special cases, and there is no sequence in his reasoning. A 

 nation may be rushing to ruin, bin that does not prevent an in- 

 dividual from growing rich. Phosphates and nitrates may be 

 diminishing, but that does not prevent them from accumulating 

 on any particular farm. We traverse Dr. Grififiths's statement 

 without qualification, that the farmer who uses nothing else but 

 farmyard manure exhausts his land. We believe he improves, 

 his land. — The Reviewer.] 



