338 PROGRESS OF SCIENCE IN THE CENTURY. 



wing of a flying bird is analogous with that of a 

 butterfly, for both are organs of true flight, which 

 strike the air ; but they are not homologous, for there 

 is no resemblance in their structure or development, 

 (c) Thirdly, the wing of a flying bird is both homolo- 

 gous and analogous with the wing of a bat. 



It must not be supposed that the question is so 

 easy as the illustrations given may suggest. Indeed 

 there are few questions more difficult than the cri- 

 teria of homology. But the importance of the dis- 

 tinction which Owen drew is obvious, for a true or 

 natural classification which groups related forms to- 

 gether must be based on the demonstration of homol- 

 ogies. Perhaps the most important addition to 

 what Owen said is due to Professor Ray Lankester 

 who, in 1870, distinguished homogeny (correspond- 

 ence due to common descent) from homoplasty (cor- 

 respondence due to similar adaptations in unrelated 

 forms). 



Starting again from Goethe, we might, if space 

 permitted, seek to show how the morphology of 

 plants developed through the labours of Schleiden 

 (1804-1881) the title of whose text-book (1842-43) 

 Botany as an Inductive Science struck a new note, of 

 Von Mohl, of Carl von Nageli, of Hofmeister, who 

 from 1849 onwards did for the pedigree of plants 

 what Gegenbaur, Huxley, and others did for animals, 

 of Robert Brown, Irmisch, Hanstein, Alex. Braun, 

 and many more. From these through De Bary and 

 Sachs, we pass naturally to the active botanical mor- 

 phologists of to-day. 



It may be more useful to try to illustrate some of 

 the more general steps in the progress of morphology. 



The first edition of Herbert Spencer's Principles 

 of Biology and Ernst Haeckel's Generelle Morpho- 



