492 



NATURE 



{Aprils, 1877 



The tail is likewise curved up underneath, and lies with 

 its broad surface towards the body, turning either towards 

 the right or the left, and thickening part of the hinder 

 extremities. In three examples the extremities are fully 

 developed, and even show the characteristic discs on the 

 tops of the toes. In the fourth example all four extremi- 

 ties present short stumps, and as yet show no traces of 

 toes, whereas, as is well known, in the Batrachia afitira 

 generally the hinder cxtremi'ies and the ends of the feet 

 first appear. Neither of branchiae nor of branchial slits 

 is there any trace. On the other hand, in the last-men- 

 tioned example, the tail is remarkably larger, and has its 

 broad surface closely adherent to the inner wall of the 

 vesicle, and very full of vessels, so that there can be no 

 doubt of its function as a breathing organ. As develop- 

 ment progresses, the yelk-bag on the belly and the tail 

 become gradually smaller, so that at last, when the little 

 animal, being about 5 mill. long, bursts through the enve- 

 lope, the tail is only r8 mill, in length, and after a few 

 hours only 0*3 mill, long, and in the course of the same 

 day becomes entirely absorbed. Examples of the same 

 batch of ova, which were placed in spirit eight days after 

 their birth, have a length of from 7'o to 7"5 mill, whence 

 we may conclude that their growth is not quicker than in 

 other species of Batrachians. 



The development of this frog. Dr. Peters observes (and 

 probably of all the nearly allied species), without metamor- 

 phosi-, wiihout branchiae, with contemporaneous evolution 

 of the arterior and posterior extremities, as in the case of 

 the higher vertebrates, and within a vesicle, like the amnion 

 of these latter, if not strictly equivalent to it, is truly re- 

 markable. But this kind of development is not quite 

 unparalleled in the Batrachians, for it has long been known 

 that the young of Fipa americana come forth from the 

 eggs laid in the cells on their mother's back tailless and 

 perfectly developed. In them, likewise, no one has yet 

 detected branchia;, and we also know from the observa- 

 tions of Camper,' that the embryos at an earlier period are 

 provided with a tail-like appendage, which in this case 

 also, may be perhaps regarded as an organ of breathing, 

 possibly corresponding to the yelk-placenta of the hag- 

 fish. As regards this point, aho, Laurenti says of the 

 Fipa : " Pulli ex loculamentis dorsi prodeuntes, metamor- 

 phosi nulla.?" (S>n. Rept, p. 25.) 



It M'ouW be of the highest mterest, Dr. Peters adds, to 

 follow exactly this remarkable development on the spot. 

 The development of the embryo of these Batrachians in a 

 way very like that of the scaled Rep'ilia makes one suspect 

 that an examination of the temporary embryonic structures 

 of Hylodes and Pipa would result in showing remarkable 

 differences from those of other Batrachians. 'T'he general 

 conclusions which miKht be drawn from this discovery are 

 so obvious, says Dr. Peters, in conclusion, that it would be 

 superfluous to put them fortvard. 



A subsequent communication of Dr. Peters to the 

 Academy informs us that it had escaped his notice that 

 M. Bavay, of Guadaloupe, had already published some 

 observations on the development of Hylodes inartini- 

 censis? .■\ccording to his observations, on each side of 

 the heart there is a branchia conS'Sting of one simple 

 gill-arch, which on the seventh day is no longer discern- 

 ible. On the ninth day there is no longer a trace of a 

 tail, and on the tenth day the little animal emerges from 

 the egg. M. Bavay also observed the contemporaneous 

 development of the four extremities, and hints at the 

 function of the tail as an organ of breathing. 



The observations of Dr. Gundlach, therefore, says Dr. 

 Peters, differ in some respects from those of M. Bavay. 

 It would be specially desirable, however, to ascertain 

 whether tre arched vessel on each side of the heart is 

 really to be regarded as a gill-arch, or only as the in- 

 cipient bend of the aorta. 



• (:omm Soc Keg. Getting. CI. phys. ix p. 135 (1788). 

 Ann, trc. Nat. scr. 5, xvii., art. No. j6 (1873.) 



TYPICAL LAWS OF HEFEDITY^ 

 \^7'E are far too apt to regard common events as 

 * * matters of course, and to accept many things as 

 obvious truths which are not obvious truths at all, but 

 present problems of much interest. The problem to 

 which I am about to direct attention is one of these. 



Why is it when we compare two groups of persons 

 selected at random from the same race, but belonging to 

 different generations of it, we find them to be closely 

 alike ? Such statistical differences as there may be, are 

 always to be ascribed to differences in the general con- 

 ditions of their lives ; with these I am not concerned at 

 present, but so far as regards the processes of heredity 

 alone, the resemblance of consecutive generations is a 

 fact common to all forms of life. 



In each generation there will be tall and short indi- 

 viduals, heavy and light, strong and weak, dark and pale, 

 yet the proportions of the innumerable grades in which 

 these several characteristics occur tends to be constant. 

 The records of geological history afford striking evidences 

 of this. Fossil remains of plants and animals may be dug 

 out of strata at such different levels that thousands of 

 generations must have intervened between the periods in 

 which they lived, yet in large samples of such fossils we 

 seek in vain for peculiarities which will distinguish one 

 generation taken as a whole from another, the different 

 sizes, marks and variations of every kind, occurring with 

 equal frequency in both. The processes of heredity are 

 found to be so wonderfully balanced and their equi- 

 librium to be so stable, that they concur in maintaining 

 a perfect statistical resemblance so long as the external 

 conditions remain unaltered. 



If there be any who are inclined to say there is no 

 wonder in the matter, because each individual tends to 

 leave his like behind him, and therefore each generation 

 inust resemble the one preceding, I can assure them that 

 they utterly misunderstand the case. Individuals do not 

 equally tend to leave their like behind them, as will be 

 seen best from an extreme illustration. 



Let us then consider the family history of widely dif- 

 ferent groups ; say of 100 men, the most gigantic of their 

 race and time, and the same number of medium men. 

 Giants marry much more rarely than medium men, and 

 when they do marry they have but few children. It is a 

 matter of history that the more remarkable giants have 

 left no issue at all. Consequently the offspring of the 100 

 giants would be much fewer in number than those of the 

 medium men. Again these few would, on the average, 

 be of lower stature than their fathers for two reasons. 

 First, their breed is almost sure to be diluted by 

 mnrriage. Secondly, the progeny of all exceptional 

 individuals tends to "revert" towards mediocrity. Con- 

 sequently the children of the giant group would not 

 only be very few but they would also be compa- 

 ratively short. Even of these the taller ones would be 

 the least likely to live. It is by no means the tallest men 

 who best survive hardships, their circulation is apt to 

 be languid and their constitution consumptive. 



It is obvious from this that the 100 giants will not 

 leave behind them their quota in the next generation. 

 The 100 medium men, on the other hand, being more 

 fertile, breeding more truly to their like, being better fitted 

 to survive hardships, &c., will leave more than their pro- 

 portionate share of progeny. This being so, it might be 

 expected that there would be fewer giants and more 

 mvdium-sized men in the second generation than in th^ 

 first. Yet, as a matter of fact, the giants and medium- 

 sized men will, in the second generation, be found in the 

 same proportions as before. The question, then, is 

 this : — How is it that although each individual does }iot 

 as a rule leave his like behind him, yet successive genec 

 tions resemble each other with great exactitude in 

 their general features ? 



' Lecture delivered at the Royal Institution, Friday evening, Feb) 

 9, by Francis Gallon, F R. S. 



