io SCOPE OF COMPARATIVE MORPHOLOGY 



less degree after propagation by seed : and that thus the possibility has 

 been demonstrated of origin and establishment of new forms from more 

 or less dissimilar parents. This is not the place to discuss the sources of 

 variation : whether it arises by a cumulative summation of slight differences, 

 or by mutations per sal turn, or both : nor whether the characters acquired 

 during the individual life are or are not transmitted to the offspring, thus 

 giving a positive direction to variation : nor yet to consider the effect of 

 sexuality, and of the subsequent reducing-division of the nucleus in dis- 

 tributing the qualities inherited from the parents. It suffices for our 

 theoretical position that variations do occur, and that they are liable to 

 be transmitted to offspring. The struggle for existence in this greatly 

 over-populated world necessarily acts as a sieve upon such variants, and 

 though the survival of the fittest is in no sense a positively constructive 

 factor in itself, it results in the preservation of what is capable of self-support, 

 and the elimination of what is physiologically less efficient. But when 

 thus much is granted, it amounts only to this : that living organisms 

 demonstrate that such an origin as evolutionary theory contemplates is 

 at least possible. It does not necessarily follow that all known forms did 

 originate in this way. Still, we are justified in accepting this view as a 

 " working hypothesis," much more probable than any other explanation 

 hitherto given of the existence of various living forms. 



But though we may readily adopt an evolutionary view, as a working 

 hypothesis applicable to organic forms at large, it is when we apply it in 

 detail that the real difficulties begin. We contemplate, for instance, some 

 group of plants which have essentially similar form, structure, and develop- 

 ment : we find that they differ in certain details and proportions, and 

 that it is possible to lay them out in a series extending from one extreme 

 form, through minor gradations, to another extreme form. Such a series 

 may be strengthened by tracing parallelism of variations of two or more 

 characters. Where this can be done the probability of the series representing 

 a real evolutionary line is greatly enhanced. But there are at least three 

 ways in which such a series may be read: (i) that the simplest form was 

 the most primitive, and the whole series one of progression : (2) that the 

 most complex was the most primitive, and the whole series one of reduction : 

 (3) that the origin was from some central point, and the development 

 divergent in tu'o or more directions. Any one of these alternatives would 

 be compatible with general evolutionary probability. How are we to 

 decide which to adopt in any given case ? 



The general principle that progress has been from the simpler to the 

 more complex gives to the first alternative a prima facie probability. As 

 a matter of fact this ' consideration weighed largely in the phylogenetic 

 decisions of a quarter of a century ago, and the opinions on the descent 

 of Ferns serve as a good illustration of it. Those Ferns which havi 

 the smallest sporangia (Polypodiaceae, Hymenophyllaceae) were held to 

 be the most primitive, while those with larger and more complex sporangia 



