THEIR COMPARATIVE BASIS 235 



In cases where there is good reason to believe that the phyletic origin 

 is correctly recognised, and where the type is represented by numerous 

 well-known species, a very strong presumption may be accepted, amounting 

 almost to a demonstration, of what has taken place in the more recent 

 steps of descent. This is more easily illustrated in respect of a given 

 part, than of the whole organism. For instance, in the phyllodineous 

 Acacias the progressive amplification of the phyllode and the progressive 

 reduction of the lamina are practically demonstrated by comparison of 

 the various species included in the single genus : the conclusion is 

 further supported by the facts of development of the individual seedling ; 

 for the young plants frequently show in their ontogeny the steps which 

 comparison among distinct species had already suggested. It is unnecessary 

 to multiply examples of such phenomena, for they are familiar to every 

 student. 



It is, however, the familiarity with such ideas, in cases where sufficient 

 evidence is available (a condition frequently seen among the Higher 

 Plants), which has led to their misuse in cases where the evidence is 

 less complete. Where ordinal or generic types are isolated, and the 

 genera represented, it may be, by few species, or even by a single one, 

 as is so often the case in the Pteridophytes, the weapon of comparison 

 is apt to lose its temper and its edge. Still, it has been used, but 

 in these isolated cases the comparative argument is less cogent, its 

 application being more violent and less exact. The cogency of all 

 morphological comparisons varies inversely with the distinctness of the 

 organisms compared : this is especially to be borne in mind in dealing 

 with questions of progressive amplification or reduction among the 

 Archegoniatae.* 



Looking back upon the theories of amplification or of reduction which 

 have been suggested in the past, it becomes evident that they have often 

 been applied at random. That one or the other has been advanced 

 according to the taste, or, one might almost say, according to the tempera- 

 ment of the writer : frequently they have been invoked under the pressure 

 of doubt, or in support of an insecure hypothesis. More especially was 

 this so in the days when monophyletic views ruled more than they at 

 present do. A full recognition of the probability of polyphyletic origins 

 has obviated the necessity which was once felt to refer all related 

 organisms to one scheme : there is no present obligation to explain their 

 form as derivative from one type, either by amplification, or by the more 

 common deus ex machina reduction. 



Goebel has drawn attention to the prevalence in phyletic speculation 

 of theories of arrest of development over theories involving amplification. 

 He remarks that most of our phylogenetic series- are reduction-series, 1 

 and traces this to the fact that a definite type is habitually recognised as 

 a starting point for comparison. Naturally such a type must already be 



1 Organography ', Part I., p. 60. 



