October 21, 1904.] 



SCIENCE. 



525 



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 rep- 

 resentative 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 gen- 

 erations. The right hand of a child re- 

 peats 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 individual identity of parts 

 of successive generations is not common. 

 It may be traced, for instance, in the coty- 

 ledons, and the first plumular 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 individ- 

 ual is liable to be indefinitely increased, 

 while often the absence of strict rule in 

 their relative positions makes their iden- 

 tical 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 in- 

 dividual identity, but their similarity in 

 other respects; and when we group them 

 imder the same head we recognize, not their 

 strict homogeny according to the definition 

 of Lankester, but their essential corre- 

 spondence, as based xipon 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, which are as a rule definite 

 neither in number nor in arrangement, 

 and are subject to variation in both re- 

 spects, according to the conditions which 



may be imposed upon them by experiment; 

 nevertheless they accurately maintain their 

 structural characters, and their essential 

 correspondence is thus established, but not 

 their individual identity. It is clear that 

 this is a comparison of a more lax order 

 than the recognition of their individual 

 homogeny would be. 



But if room for doubt of the strictest 

 homogeny be found in simple cases such 

 as these, what are we to expect from the 

 comparisons of less strictly similar parts 

 of the plant, such as cotyledons, scale 

 leaves, foliage leaves, bracts, sepals, petals, 

 stamens, carpels? How far are these to 

 be held to be homogeneous, or in some 

 less strict sense homologous? Or, going 

 still further, how are we to regard those 

 comparisons which deal with parts of dif- 

 ferent individuals, species, genera, orders 

 or classes? What degree of homology is 

 to be accorded to them ? In proportion as 

 the systematic remoteness of the plants 

 compared increases, and the continuity of 

 the connecting forms is less complete, so 

 the comparisons become more and more 

 doubtful, and the use of the term 'homol- 

 ogy' as applied to them more and more 

 lax, until we are finally landed in the 

 region where compai-ison is little better 

 than surmise. It becomes ultimately a 

 question how far the term 'homology' is 

 to be held as covering these more lax com- 

 parisons, which are certainly not examples 

 of 'homogeny' in Lankester 's sense, and 

 are only doubtfully correlated together on 

 a basis of comparison of more or less allied 

 forms. 



The progress of our science should be 

 leading towards a refinement of the use of 

 the term 'homology'; an approach must 

 be made, however distant it may yet be, 

 to a classification of parts on a basis of 

 descent. But though that may be readily 

 accepted in theory, it is still far from be- 

 ing adopted in the general practise of 



