64 PLANT MORPHOLOGY 



together were or were not comparable as regards their descent. For 

 long years after the publication of the Origin of Species homology had 

 no evolutionary significance in the practice of plant morphology. 

 But in the sister science of zoology this matter was taken up by Ray 

 Lankesterin 1870, in his paper "On the Use of the Term Homology 

 in Modern Zoology, and the Distinction between Homogenetic and 

 Homoplastic Agreements." Many botanists at the present day 

 would be the better for a careful study of that essay. He pointed 

 out that the term homology as then used by zoologists belonged to 

 the Platonic school, and involved reference to an ideal type. This 

 meaning lay at the back of Goethe's theory of metamorphosis in 

 plants, and it seems to have been somewhat in the same sense that 

 homologies were traced by Hofmeister. Lankester showed that the 

 zoologists' use of the term "homologous" included various things: 

 he suggested the introduction of a new word to define strict homology 

 by descent: structures which are genetically related in so far as 

 they have a single representative in a common ancestor, he styled 

 "homogenous;" those which correspond in form, but are not genet- 

 ically related, he termed "homoplastic." 



It is important at once to recognize that the strict "homogeny" 

 defined by Lankester as that of "structures which are genetically 

 related in so far as they have a single representative in a common 

 ancestor" can only be traced in the simpler cases of plant-form: it 

 implies the repetition of individual parts, so strictly comparable in 

 number and position as to stamp the individual identity of those 

 parts in the successive generations. The right hand of a child repeats 

 in position and qualities the right hand of the mother, and of the race 

 at large: here is a strict homogeny. In the plant-body such in- 

 dividual identity of parts of successive generations is not common. 

 It may be traced, for instance, in the cotyledons, and the first plumu- 

 lar leaves of seedlings of nearly related species, or in their first roots. 

 But as a consequence of that continued embryology, which is so 

 constant a feature in the plant-body, the number of the appendages 

 of any individual is liable to be indefinitely increased, while often 

 the absence of strict rule in their relative positions makes their 

 identical comparison in different individuals impossible. This is 

 especially clear in the case of roots of the second, and higher orders, 

 for they do not correspond in exact number or position in seedlings. 

 What we recognize in such cases is, then, not the individual identity: 

 but their similarity in other respects: and when we group them 

 under the same head we recognize, not their strict homogeny accord- 

 ing to the definition of Lankester, but their essential correspondence, 

 as based upon the similarity of their structure, and of their mode of 

 origin upon and attachment to the part which bears them. This is 

 also the case with the antheridia and archegonia of the pteridophytes. 



