378 



SCIENCE. 



[N. 



Vol. VIII. No. 195. 



ture of its sporophyte body. Any attempt 

 to relate these to one another upon the 

 basis of a single structure, even so impor- 

 tant a one as thesporogonium, is essentially 

 misleading. But when we consider the 

 totality of structure we are led to the opin- 

 ion that these lines possibly diverged from 

 an archetypal plexus in which there were 

 gametophyte bodies as simple as that of 

 Aneura and sporophyte bodies as simple 

 as that of Eiccia. Another illustration is 

 the recent attempt of Arnoldi to associate 

 Isoetes with Selaginella largely upon the 

 basis of endosperm development, without 

 regard to great diversities in habit and 

 anatomical details. The association may 

 be perfectly proper, but the reason given 

 for it is inadequate. 



lu dealing with problems of phylogeny it 

 is also important to remember that the 

 origin of a prominent group of living forms 

 from another group of living forms is ex- 

 tremely improbable. We can point out 

 resemblances in structures which we have 

 come to regard as essential, but this is not 

 likely to mean the origin of the one group 

 from, the other. It may mean that the two 

 groups can be traced to one, probably now 

 extinct, which combined the characters now 

 differentiated. Most living groups are best 

 regarded as divergent rather than consecu- 

 tive series. 



But even this apparently sure ground has 

 become very uncertain from the fact, becom- 

 ing more and more apparent, that similar 

 changes in structure, even very important 

 ones, may have appeared independently in 

 different lines. The response of organisms 

 in structure to their environment is deeper 

 seated than we were once inclined to believe, 

 and testimony from the similarity of certain 

 structures, when contradicted by the major- 

 ity of other structures, argues feebly for 

 recent community of origin. Such similari- 

 ties in structure argue more for physiolog- 

 ical conditions than for phylogeny. For 



instance, from the standpoint of evolution, 

 the appearance of heterospory among the 

 pteridophytes is one of the most important 

 contributions to plant progress made by the 

 group, but it is impossible to escape the 

 conclusion that heterospory was attained 

 independently by several lines. To put 

 into the same genetic group all heter- 

 osporous pteridophytes would be regarded 

 as a morphological absurdity. If heter- 

 ospory appeared independently in several 

 lines the same conclusion must be reached 

 in reference to its natural outcome, the 

 seed, and the polyphyletic origin of the 

 spermatophytes becomes extremely prob- 

 able. 



This increases the perplexities of phylog- 

 eny, but it broadens its horizon and intro- 

 duces another possibility. To continue the 

 same illustration, in our search for the 

 origin of seed-plants we have narrowed 

 attention to the existing heterosporous 

 pteridophytes, when some of the spermato- 

 phyte groups, as, for example, the gymno- 

 sperms, may represent an entirely distinct 

 line in which heterospory and then the 

 seed appeared, and may not be related di- 

 rectly to any existing heterosporous pteri- 

 dophyte. In such a case we are permitted 

 to look to some group of living homospo- 

 rous pteridophytes as possibly containing 

 the best living representatives of the group 

 from which gymnosperms have been de- 

 rived. 



With all these possibilities in mind, I 

 wish to discuss the phylogeny of the gym- 

 nosperms, not so much to reach a clear 

 phylogeny as a clearer understanding of 

 the complexity of the problem and the 

 uncertainty of conclusions. This is a field 

 in which no one can afford to be dogmatic. 



THE ORIGIN OF GYMNOSPEEMS. 



From Hofmeister's classic researches to 

 the discovery of gymnosperm spermatozoids 

 by Hirase, Ikeno and Webber, the fact has 



