528 



NATURE 



[Oci. 2, i8;9 



111 Dr. Pole's typical case of dichromic vision, the sensation 

 which is absent is the middle one, and the two existing sensations 

 are not both identical with those in normal eyes, but have their 

 maximum impressions produced by vibrations whose rates are 

 nearer together, so as not to leave the retina insensible to the 

 middle of the spectrum. His long-wave colour corresponds 

 with the sodium line D, which is therefore a primary to him, 

 and his short-wave primary is probably violet ; green light, 

 accordingly, causes very little sensation, or will appear yellowish 

 or bluish, as its vibrations come within the scope of one or other 

 of the two existing sets of nerves. 



It appears to me, i priori, that it is not unlikely, in the case 

 of the absence of the intermediate (green) sensation, that the 

 other two sensations should become approximated, so as to be 

 of more use in discriminating a continuous series of colours ; and 

 such an " equiUbration " is analogous to the hyper-sensitiveness 

 of touch or hearing to which a blind man attains, one channel 

 of communication with the external world the more readily 

 lending its aid when another is closed. 



This hypothesis of individual differences in the primary colour- 

 sensations seems satisfactorily to remove the apparent violence 

 done to the facts of dichromic vision by the Young-Helniholtz 

 theory, and to accord with all other evidence of the physical 

 basis of perception of colour. A. H. 



Prof. Mivart on "Tails" 



In the Davis lecture (Nature, vol. xx. p. 509), Prof. St. George 

 Mivart remarks that kangaroos use their tails " to a certain extent 

 in their long jumps." This may either mean that the tail is used 

 as a balance to the fore-part of the body — as, of course, is the 

 case — or as a means of propulsion. Tiie latter is the natural 

 inference to be drawn from the sentence. 



The belief that kangaroos thus actively employ their tails is of 

 wide extent in Australia, but a residence there of over three 

 years, principally in the bush, so thoroughly convinced me that 

 the idea was erroneous that I think I am justified in challenging 

 the Professor, if I have not misinterpreted him, to give the 

 evidence on which his statement is based. 



As we have recently learnt, through photographs of a galloping 

 horse (vide Field, June 28, 1879), eye observations, where move- 

 ments are rapid, are not entitled to much weight ; but still, as I 

 have seen many hundred kangaroos pass at full speed, and have 

 observed them especially in reference to this question, some little 

 dependence may be placed on my assertion that the action of one 

 of these animals in full stride is incompatible with the use of its 

 tail as a third hind leg. 



In the descent of the kangaroo in each leap, the tail swings 

 freely upwards to a curve, whose arc is at about right angles to 

 the slope of the back, and during the rise of the succeeding leap 

 it falls, and is then apparently impeded by muscular action. 

 Were the tail actively employed, it would have to strike the 

 ground almost at the same moment as do the feet, and the whole 

 form of the animal would be altered. The tail would have to 

 be brought down rapidly, like a riding whip, whilst the upward 

 swing would be retarded. 



From the forward slope of the body in long leaps and the 

 fact that then, of the hind limbs, only the feet touch the ground, 

 it is evident that the portion of the tail that alone could be em- 

 ployed as a propeller would be the posterior and weaker part. 



Again, if we consider the speed of the kangaroo over the 

 ground — say, 15 miles an hour, or 7J yards in a second — we find 

 that only length of bones and mobility of joints of the hind legs 

 permit the feet to rest on the same spot of ground for even a 

 fraction of a second. With the tail it is otherwise ; in the 

 position it is carried by the animal it mu^t travel over the surface 

 for some two feet, though but pressed against the ground for a 

 tenth of a second. I fear not even the callous under-surface of 

 a kangaroo's tail would long stand such terrible attrition. 



If further evidence is wanted, it is to be found in the tracks 

 left by kangaroos, wallaby, &c., that have travelled quickly over 

 sand, mud, &c. There is seldom to be seen the mark of a tail, 

 and then only as a graze. The very tendons of the tail ai'e an 

 argument against its utility in leaping, for those on the upper 

 side are twice as powerful as those on the under. 



In conclusion let me say that it has been suggested to me that 

 the remark about kangaroos making use of their tails to carry 

 grass might lead to the supposition that their tails are prehensile. 



E. H. Pringle 



Beckenham, September 29 



In Prof. Mivart's interesting lecture on "Tails" the state- 

 ment occurs that kangaroos " make use of their tails to a certain 

 extent in their long jumps." This I believe to be an entire mis- 

 apprehension. No doubt the massive tail of a kangaroo suggests 

 the possibility of such use, and the idea is helped out by the very 

 obvious employment of its tail by the animal in almost all its slow 

 movements, when it rests of necessity on the ground. In leading, 

 however, the body is thrown forward at an angle which raises 

 the tail from the earth. On the great sandy flats abaut the 

 shores of Moreton Bay, kangaroos have often afforded mc the 

 opportunity of convincing myself that the tail touches the earth 

 only occasionally and very lightly, probably when the balance 

 of the body is not perfectly maintained. I have traced the 

 marks of the great hind toes over this sand when damped by 

 rain, and in the best condition to take the slightest impression, 

 and have never found anything more than ^very faint mark now 

 and then, and that evidently such as had not been produced by 

 any effort of the animal to urge itself onward by means of its 

 tail. In one instance the track of a kangaroo, which had crossed 

 a flat within sight of me at full speed, happened to cut the track 

 of some small wading bird, which enabled me to compare the 

 deep holes made by the powerful stroke of the hind legs with 

 the shallow oval mark made by the tail, about half a dozen times 

 only in a distance of about four hundred yards, and the track of 

 the bird. Had any propulsive force been used, this mark must 

 have been very distinct on a surface capable of taking a perfect 

 impression of the bird's foot. The rock kangaroos leap in 

 exactly the same manner as their larger congeners, yet their long 

 and comparatively slender tails could hardly be supposed by any 

 one to render them any assistance. Even in the confined space 

 .allotted to them in the Zoological Gardens it may be seen that 

 kangaroos leap without any help from their tails, and when they 

 are going very fast the axis of the body is thrown forward i nto a 

 position approaching the horizontal, and the tail is then quite 

 clear of the ground. Prof. Mivart is not singular in his misap- 

 prehension on this point, for I find in Prof. Alleyne Nicholson's 

 "Manual of Zoology," edition 1875, p. 580, that "the tail is 

 also extremely long and strong, and by the assistance of this 

 organ and the powerful hind limbs, the kangaroos are enabled 

 to effect extraordinarily long and continuous leaps." 



Arthur Nicols 



About Snakes 



How do snakes progress ? I ask this question in full know ledge 

 of all that has been written on the subject, and nought that I 

 have read satisfies my mind nor meets all requirements. The 

 books tell us that the ribs of the snake are its legs. All well 

 and good, as long as we are dealing with smooth plane surfaces 

 and sluggishly moving snakes, as the boas ; but the theory v tterly 

 fails with snakes which literally fly through grass, or climl trees 

 or walls with equal facility. On one occasion I allowed a boa 

 to pass over my hand pressed flat on the ground, and Idislinctly 

 felt the onward movement of the rib legs, acting exactly like the 

 outside legs of the centipede, and I felt quite satisfied that, under 

 the circumstances, the snake was moving by means of its ribs. 



But this timid and harmless rat snake, which you see absolutely 

 flying through the grass to escape you, cannot be moving by the 

 aid of its ribs ; for first, there is nothing for them to acto;i, and 

 secondly, you cannot imagine the ribs acting rapidly enough to 

 ensure the immense speed at which the animal passes th rough 

 the high grass. Look at this tree snake surveying you from a 

 bough ; how did it get there ? The books will tell you, by w inding 

 itself round the tree, and thus progressing upwards ; but a 

 slender snake of 12 — 18 inches in length cannot grasp a trank 

 2 feet in diameter for the first start up the tree, and so it must 

 get up some other way. 



How did this little snake 9 inches long ascend the side of a 

 glass jar lo inches high, and assume the position I shall presently 

 describe ? Certainly its rib legs gave it no assistance. 



The man who brought the boa, had with him an earthen pot 

 full of small ash-coloured snakes, quite new to me, and I never 

 saw ferocity and activity more remarkably combined than in 

 those small reptiles ; on taking them out by the handful, they 

 fastened their jaws (well armed with teeth) upon the man's hand 

 and wrist, and hung like gigantic leeches. Accustomed to such 

 attacks, the snake owner removed the little demons, each sepa- 

 rately, with some difficulty, and the wounds bled freely. But 

 the activity, and 'mode of progression in these snakes inter- 

 ested me more than their ferocity ; thrown on the ground they 



