APPENDICES 1445 



We give prominence, which some may perhaps regard as exag- 

 gerated, to Steenstrup's elucidation of "alternation of generations" 

 in animals, especially in Hydroids and Trematodes. It may be defined 

 as the alternate occurrence in one life-cj^cle of two or more different 

 forms differently produced. Thus a practically asexual Campanu- 

 larian Hydroid colony, very plant-like in appearance, buds off free- 

 swimming Medusoids which produce eggs and sperms. The fertilised 

 eggs develop into free-swimming larva?, which eventually settle down 

 and begin the asexual budding which results in a zoophyte colony. 

 Of this discovery there had been anticipations, such as the poet 

 Chamisso's observation, early in the nineteenth century, of the 

 alternation of generations in the remarkable free-swimming Tunicate 

 called Salpa, but the generalisation of a large number of other 

 puzzling phenomena as kindred ones was to Steenstrup's credit. 

 But part of the reason for giving prominence to alternation of 

 generations is because in Hofmeister's hands (1849-51) the idea 

 served to unify in a new and luminous way the life-history of the 

 long series of plants from liverworts to flowers. By a brilliant series 

 of researches, perhaps unsurpassed in the history of biology, Hof- 

 meister traced the regular alternation of a sexual and a spore- 

 producing phase in the life-cycle — an alternation between a sexual 

 "gametophyte" and an asexual "sporophyte". Thus the asexual 

 fern-plant produces spores; these develop on the damp earth into 

 minute sexual prothalli, from the fertilised ova of which the 

 familiar "fern-plants" develop. But to follow this clue into the deeply 

 masked alternation that obtains in flowering plants was an achieve- 

 ment of the first rank — so brilliant indeed that it was hardly a 

 surprise when long afterwards there came the demonstration of 

 motile spermatozoa in the pollen-tubes of C3^cas and Gingko, which 

 are primitive Flowering Plants. 



In 1892, in his Germ-Plasm and in earlier papers, August Weis- 

 mann (1834-1914) made a great advance in the study of heredity. 

 In his exposition of "the continuity of the germ- plasm" he 

 explained, for the first time clearly, how it is that like tends to beget 

 like; he focused the fact of "germinal variations", which was a 

 distinctly post-Darwinian idea; he anticipated the modern theory 

 of the chromosomes as vehicles of the hereditary "factors" or 

 "determinants"; and he subjected to a searching and damaging, if 

 not fatal, criticism the evidence adduced in support of the trans- 

 missibility of individually acquired somatic modifications. In regard 

 to all these Weismann had, as usual, his anticipators; but it is due 

 to him to say that he thought out a coherent theory of heredity. 



Another great initiator was Louis Pasteur, chemist first and 

 biologist later, who advanced logically from his first study of tar- 

 trates not only to a recognition of the manifold activities of bacteria, 

 but to some prevision of the part that yeast and other living "fer- 



