September, igo6.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



531 



For 35 years his work, although of such fundamental 

 importance, remained entirely unrecognised, until in 

 190Q it was rescued from its obscurity by the simul- 

 taneous discoveries of de Vries, Correns, and 

 Tschermak. It was mainly by the simplicity of his 

 experiments, in which the inheritance of some particu- 

 lar character was traced through several generations, 

 that Mendel was able to arrive at the hypothesis which 

 has revolutionised our conceptions of the physiology of 

 heredity. Mendel's work was carried out by means of 

 artificial fertilisation in a race of plants chosen, after 

 careful preliminary work, as suitable for the purpose 

 for which it was used. 



" The experimental plants must necessarily : — 

 " (i) Possess constant differentiating characters. 

 " (2) The hybrids of such plants must, during the 

 flowering period, be protected from foreign 

 pollen, or be easilv capable of such pro- 

 tection. 

 " (3) The hybrids and their offspring should suffer 

 no marked disturbance in their fertility in 

 the successive generations." " 

 Such plants Mendel found in the Garden Pea (Pisum 

 sativum and its varieties). From among the many 

 varieties of peas he chose pairs of varieties, for artificial 

 cross fertilisation, in such a way that the members of 

 each pair differed from one another in some one definite 

 character. The characters selected for experiment 

 were : — (i) The shape of the ripe seed, whether round 

 or deeply wrinkled; (2) the colour of the cotyledons in 

 the ripe seed (i.e., the colour of the seed after the re- 

 moval of the seed coat); whether green or yellow; (3) 

 the colour of the seed coat, whether white or some shade 

 of brown; (4) the shape of the ripe pod, whether simply 

 inflated or deeply contracted between the seeds; (5) 

 colour of the unripe pod, whether green or yellow; 

 (6) the position of the flowers, whether distributed along 

 the stem, or crowded near the top; (7) the length of the 

 stem, whether 6 to 7 feet or f to i^ feet. 



Each pair of the above characters were united by 

 cross fertilisation. Mendel found that in each case 

 one only of the two characters appeared in the hybrid 

 offspring, the hybrid character resembling that of one 

 of the parental forms so closely that " the other either 

 escapes observation completely, or cannot be detected 

 with certainty. This is found to be true whichever 

 variety be used as the seed parent; that is, reciprocal 

 crosses give identical results." The character which 

 thus appears was termed by Mendel the dominant, the 

 character which is hidd(;n being the recessive. 



The next generation is obtained by self-fertilising 

 these hybrid plants. The result of this is that in these 

 peas the offspring, instead of being all alike, break 

 into two forms, resembling re.'-'pectively the two parental 

 forms. The ratio in which these two forms are ob- 

 tained is, within the error of observation, that of thnc^ 

 dominants to one recessive. 



Take, for example, Mendel's second experiment of 

 crossing two varieties differing from one another in the 

 colour of the cotyledons. The cotvlcdons are, of course, 

 the first leaves of the young plants, so that this particu- 

 lar cross has the advantage that th(> characters of the 

 offspring can be detcrmini'd in the same season that 

 the cross is made. 



Mendel made 58 crosses, yielding 253 seeds, all of 

 wliiiii 1i:k1 yellow cotyledons. R. P. Ci. 



f To be conlinvid. 1 



•Bateson's trans'ation of Mendel's orlRinal paper. See 

 "Mendel's Principlei of Ilerpditv " ("nmli I'niv Pres?. 

 Page^a. 



The Origin of Birds. 



By \V. P. PvcRAiT, A.L.S., F.Z.S. 



That birds and reptiles, in spite of their apparent 

 differences to-day, are really very closely allied, admits 

 of no dispute. A comparison of the skeletons alone of 

 the two types would be sufficient to demonstrate this, 

 while the brain, vascular, and urino-genital systems 

 furnish no less striking testimony. But, apart from 

 the indubitable evidence to be obtained from living 

 birds, other and even more striking testimony is to be 

 obtained from the remains of fossil forms. 



This evidence carries us back to Jurassic times, the 

 oldest known fossil bird — Archajopteryx — having been 

 obtained from the lithographic slate of Solenhofen, in 

 Bavaria. 



This bird more nearly resembles the reptiles than any 

 other known form. So much so, that undue and un- 

 warrantable use has been made of the fact, many 

 writers having endeavoured to show that it was more 

 reptile than bird, a contention which becomes ridiculous 

 when the facts are carefull}' considered. Two speci- 

 mens of this remarkable bird only are known — belong- 

 ing to as many species — the first to be discovered being 

 now in the British Museum, the second, and most per- 



One of tile " Pro- Avci." 



feet, in the National Ctdlection of Berlin. With the 

 specific distinctions we have nothing to do here, but 

 both agree in having the jaws armed \\ilh teeth, and a 

 long, tapering, lizard-like tail; but this, like the rest of 

 the body, bore feathers. 



Of ail the accounts that have been given of this 

 patriarch of the bird world, only one can be regarded as 

 accurate, though on many occasions one or other of 

 them have been described by men whose powers of 

 interpretation have in other ways been more severely 

 tried. It is on their descriptions that the inaccurate 

 and sometimes grotesque figures which adorn text- 

 books of comparative anatomy and natural history have 

 been based. So profoundly impressed do these authori- 

 ties appear to have been by the presence of teeth in the 

 jaws, the long tail, the armature of claws on the wings, 

 and the fact that traces remain only of the wing and 

 tail feathers, and of the feathers of the legs, that they 

 contended that these ancient types must have been 

 clothed, as to the head, neck, and trunk, with scales, 

 and as to the rest of the body with feathers. A conten- 

 tion as wildly improbable, surely, as it would be to 

 insist that, from the absence of all traces of muscles, 

 these primitive creatures had not yet acquired muscular 

 tissue. 



Nevertheless, we have, in this primitive type, not 



