288 



KNOWLEDGE 



[Kkb. 3, 1882. 



lioforo th" eclipse of 1870, when olwcrvatioiw, showing 

 what I hud alrindy tliniotistnitfd, liognn to be made. My 

 proof was not the less a proof that, if it was nut mrinrr to 

 tho general, it was not so readily understoo<l l>y them 

 as tho compamtively rough phot^jgrnpliic demonstration ; 

 and tlie lesson needs insisting upon that very <)ft<n we 

 possess, idready demonstrated, trutlis which a litth- care- 

 ful reasoning will sutlii-e to ediiec from observations 

 already niado. Tin- cry for fresh observations is, in such 

 cAses, caused either by laziness or inaptitude, either by 

 unwillingness to work out the truth from the observations 

 in hand, or by incapacity to reason soundly and accurat<>ly. 



I note that' Professor Young accepts without reservation 

 Dr. Henry Drapers important discovery of the existence 

 of oxygen in tho sun. I fully agree with him. His views 

 respecting the elementary constitution of matter he has 

 described in these pages. How far they agree (as the 

 reviewer in Xaliirr seems to suggest) with Mr. Lockyor's 

 theory —or ratlier tlie theory advanced (earlier) by Professor 

 Clarke, of Cincinnati, we leave our readers to judge. 



Professor Young's account of the valuable researches 

 made by his fellow-countryman. Professor Langley, into 

 tiie question of the sun's heat and light, is full of interest. 

 I note that Professor Young regards 10,000° Centigrade as 

 an altogether more reasonable estimate of the sun's effec- 

 tive temperature than the monstrous numbers adopted by 

 Secchi, Ericsson, and others. 



I would fain dwell longer on this valuable work and on 

 the fascinating subject of which it treats, but space will 

 not permit I recommend all who wish to know the 

 present position of solar research to take this treatise for 

 their guide- — a guide thoroughly honest and trustworthy. 

 The book is not a book to be tasted, or merely swallowed, 

 but " to be chewed and digested." It has one fault (I 

 think a somewhat serious one) : Professor Young is ex- 

 ceedingly unwilling to claim his own work ; as some of it 

 has been rather unceremoniously claimed by others, he, in 

 this, hardly does justice to those who in this countrj- have 

 urged his claims (of the justice of wliich, be it remarked, 

 he is fully conscious, little though he cares to assert them). 

 There is sucli a thing as carrying modesty too far. He also 

 fails sometimes in expressing with suHicient confidence 

 views which he is known to regard as uni|uestionably just. 

 These, however, are, after all, very amiable faults. Per- 

 haps 1 should not have noticed them if he had left me any 

 others to note. 



FOUND LINKS. 



r.v Dp. Andhew Wil.son, F li.S F, 

 PAUT HI. 



HAVING in my previous paper tried to .show that the 

 Mud fishes were veritable links between the lish- 

 e.la-ss and the frog-class, we may now turn to the history of 

 the latter group itself, by way of showing how, within its 

 own limits, gaps and gulfs have been bridged in Nature's 

 own way. The history of a frog is in itself an interesting 

 study. It begins life as a tadpole, and lives, ns most 

 readers know, a perfectly lish-lifce existence. It is fish- 

 like in form ; its heart is two-chambered, and thus 

 resembles that of the fish, and it breathes at first by out- 

 side gills. By-and-byo a liroad fold grows over the gills, 

 and ultimately covers them ; whilst internal gills grow 

 from the gill-arches. ^leanwhile, the tadpole- has been 

 cropping the waterweeds by means of the horny jaws with 

 which it is provided, and has l>een digesting its food 

 within the long and spiral intestine which is the right 



and heritage of tho vegetable feeder. Hoon, however, 

 the hind legs, which in the frogs and toads are the 

 first to appear, are developed ; and these are in turn 

 succeeded by the front limbs. Lungs l)egin also to grow, 

 as all lungs do, namely, in the form of two sacs or bags 

 from the hinder or lower wall of the gullet At this stage, 

 the likeness of the frog to the fish has disappeared, and it 

 closely resembles one of the common tailf^ "efts" or 

 newts, which are familiar denizens of our ponds and pools. 

 If it had retiiined its outside gills after its legs had become 

 developed, the young frog would have exactly resembled 

 that curious creature, the ProU^is — found only in under- 

 ground caves in Central Europe — or the curious AxolotI 

 of North America. But the ways of frog-development do 

 not permit it to remain in the guise and likeness of its eft- 

 cousins. Whilst its lungs have been undergoing develop- 

 ment, the heart has been approaching that of the frog- 

 type, which possesses a three-chambered heart, as already 

 observed. Then, as development is completed, the tail 

 shrivels. Growing " small by degrees," it is represented in 

 the adult frog by a mere rudiment ; and, as the obliteration 

 of the tail takes place, the young frog leaves the water and 

 assumes the habits of a land-existence ; breathing by lungs 

 alone in its adult state, and exchanging, moreover, the 

 vegetarian tastes of its infancy, for an insect dietary in 

 after-life. 



Now, the history of a frog is beset with questions of 

 interest for the earnest mind that studies even its .super- 

 ficial features. Why, firstly, should a frog pass through 

 these changes at all ? is a very pertinent in(]uiry ; and if 

 this be capable of being answered, why, secondly, should 

 its development run in the lines sketched out ! If we 

 start with the idea that animals and plants were simply 

 " created " as we find them — and that view of matters is, 

 of course, not yet displaced in unscientific circles — then, so 

 far as I can see, no explanation whatever of the frog's de- 

 velopment can be offered. " It is so, because it is so " — 

 such is the logical dead wall that awaits the student who 

 turns to the " special creation " theory for an explanation. 

 There is no accounting for a supernatural creative fiat ; 

 we cannot give reasons for a " special creation : " in a 

 word, we must, on this theory of nature, simply accept 

 the fact of the frog's existence, and have done with it 

 But there exists the alternative idea of evohition and 

 descent. What if it be admitted that one species or 

 group of animals arises by natural variation and descent 

 from another gi-oup 1 What if in the frog's development 

 we are led to see a panorama — a moving picture, of the 

 descent of its race ? The reasonableness of evolution may 

 thus, I think, become very apparent ; contrariwise, I 

 know of no other rational explanation of the frog's tadpole- 

 stage, and its subsequent development. 



What evolution, then, says is this : the frog is at first a 

 fish-like, gill-breathing tadpole, with a fish-heart, because I 

 its earliest ancestor was a fish ; and it is interesting to 

 note that the young of some well-known fishes (e.(!., dog- 

 fishes) breathe by outside gills. I have a beautiful spe- 

 cimen of two of these young fishes with their outside gills 

 in my museum. Furthermore, the resemblances of the 

 tadpole to the type of some primitive fish do not end with 

 its outside aspect. Mr. F. M. Balfour says the anatomy 

 of the tadpole points to its relations with the li%-ing 

 lampreys, which, as every naturalist admits, rau.st be fishes 

 of a very ancient type. But, secondly, the tailed tadpole 

 becomes four-legged, and it thus resembles, as we have 

 seen, a newt or eft The reason of newt^stage is evident 

 if we assume that the frog-stage was attained through a 

 newt-stage. Abbre%iate the tail of the newt, elongate 

 its hind legs, and with a few other modifications, we find 



