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,! 
and traces this to the fact that a definite type is habitually recognised as 
a starting point fer comparison. Naturally such a type must already be 
1 Organography, Part L., p. 60. 
