EVOLUTION AND MANKIND. 295 



Again, the disuse of their eyes by cave animals brought about 

 a dwindling and subsequent tlegeneration of these organs. Such 

 modifications or ac(iuired characters were supposed l>y Lamarck 

 to be transmissible to the »)ffs])ring. This transmissibility was 

 denied bv Weismann, but Lamarck's opinions have recently re- 

 ceived increased attention, especially in America. 



Weismann (i<S34-i9i4) propounded a hypothesis oi hered- 

 ity based on what he termed the " Continuity of the (jerm 

 Plasm."' His lxx)k on the subject was translated into English 

 in 1893. The germ cells are early separated from the body cells, 

 as can be ascertained by embryological investigations. The germ 

 cells are supposed to retain each a complete sample of the 

 ancestral germ plasm, and so to be directly continuous fr<nn 

 generation to generation, thus transmitting hereditary characters. 

 The work oif Mendel (1822-1884) sheds important light on 

 problems of heredity. His classical })aper on " Experiments in 

 Plant Hybridisation " was published in Briinn in 1865. but was 

 overlooked until about 1900. Mendel worked chiefly on crossing 

 different varieties of the garden pea. Pistim satii'uni. He found 

 that of certain contrasted couples of parental characters, which 

 did not blend, one was dominant over the other, which was latent 

 or recessive. The first generation of hybrids was apparently 

 all dominant, but it was subsequently found that they were really 

 impure dominants. When these hybrids were inbred, it was 

 found that one-c|uarter of them reverted to the dominant type, 

 one-quarter to the recessive type, while one-half reproduced 

 hybrid features, and that these proportions were maintained when 

 the impure (Kjminants were again inbred. The inbred offspring 

 of pure dominants and pure recessives bred true. 



Mendel's results indicate that small individual characters may 

 occur separately in the germ cells, atid may be transmitted 

 separatelv to the olTspring. The individual organism may, per- 

 haps, be composed of a number of factors or unit characters, with 

 regard to which there is a comjilete segregation among the germ 

 cells, each of which liears one only of each pair of contrasted 

 characters. A table of some of the dominant and recessive 

 characters of j>lants and animals was shown, for example, tall and 

 dwarf stems in peas, yellow and green cotyledons in peas, round 

 and wrinkled seeds in ])eas, susceptil)ility and immunity to rust 

 in wheat, rosecomb and single comb in fowls, eye-colour in man, 

 certain diseases in man ( such as brachydactyly, night-blindness, 

 colour-blindness, haemophilia, and pre-senile cataract). 



New varieties may arise by crossing, and some species may 

 have arisen in that manner (I>otsy). 



.'\ mode of origin of species, derived from his study - 'f 

 variations, was set forth by Professor Hugo de Vries. of Amster- 

 dam, in his work entitled " The Mutation Theory." published in 

 if^oo. His conclusions were based upon the study of plants, the 

 classical example being his work on the evening primrose, 

 Oenothera Jamarckmna. From his observations, he concluded 

 ihat species arise from one another by changes of considerable 



