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SCIENCE. 



[N. S. Vol. IV. No. 85. 



logenies, which would be vefy much easier 

 could every notable resemblance at once be 

 accepted as proof of relationship. It ofteii 

 renders impossible the proper classification 

 of some isolated genus which seems to have 

 several incompatible affinities. It empha- 

 sizes the necessity of founding schemes of 

 classification upon the totality of structure, 

 and of determining the nature of character- 

 istics, whether they are primitive or ac- 

 quired, divergent, parallel or convergent, 

 before attempting to assign them their 

 proper taxonomic value. 



We may find a practical identity in teeth, 

 skull or feet as the outcome of these pro- 

 cesses, but as yet no case is known where 

 all these structures have become alike 

 through the operation of either parallel or 

 convergent development. Among the in- 

 vertebrates the case is different. Hyatt has 

 shown that the degenerate, straight- shelled, 

 ammonoid genus Baculites is a polyphyletic 

 group, and derived from several distinct 

 stocks, both European and American. 

 Wiirtenberger points out that the so-called 

 Ammonites mutabilis is not a true species, but 

 a composite group, made up by the conver- 

 gence of several distinct lines to a common 

 term. This case is peculiarly significant, 

 because it would hardly have been detected 

 had not the embryonic and young stages of 

 the shells been preserved. 



It seems the most obvious of common- 

 places to say that numerous and close re- 

 semblances of structure are prima facie evi- 

 dences of relationship. Yet the statement 

 is true, even though the resemblances have 

 been independently acquired, because par- 

 allelism is a more frequently observed 

 phenomenon than convergence, and because 

 the more nearly related any two organisms 

 are, the more likely are they to undergo 

 similar modifications. 



All this brings us back to the thesis so 

 frequently insisted uj)on already, that the 

 only safe and trustworthy^ method of con- 



structing phylogenies is by tracing the de- 

 velopment, step by step, through all its 

 gradations ; and until this is done the clas- 

 sification of any group can be but tentative 

 and provisional, that is, if we intend classi- 

 fication to express relationship. 



No department of biological science is at 

 present the scene of such vigorous contro- 

 versy as that which deals with the factors 

 of evolution, the causes which determine 

 the development of new forms, and the prob- 

 lems of heredity which are inseparably con- 

 nected with them. Paleontological evi- 

 dence will prove to be of much importance 

 in this connection also, but it cannot well 

 have more than a corroborative value. 

 Though the examination of long and com- 

 plete phyla brings to light much that is sug- 

 gestive concerning the factors which have 

 brought these changes to pass, and any 

 rational theory must embrace and explain 

 these facts, yet the deciding weight must 

 probably come through the physiological 

 and experimental method. Time fails to 

 deal with such far-reaching questions here, 

 and yet it may be well to call attention to 

 the necessity of avoiding a dogmatic and in- 

 tolerant attitude, and to deprecate any 

 premature attempt to exclude this or that 

 class of factors from consideration. In 

 most of the recent writings upon the ef- 

 ficient causes of evolution you will find ex- 

 pressed or implied the feeling that these 

 matters are not so simple and intelligible 

 as we once supposed, and that we are yet 

 only upon the threshold of their solution. 

 The study of paleontology will not tend to 

 dispel this feeling of mystery. 



Another department of biological science 

 in which paleontology has proved of great 

 value, and will become more and more so 

 in the future, is that which deals with the 

 geographical distribution and migrations of 

 organisms. Though not a branch of mor- 

 phology, this subject has a very significant 

 bearing upon that science, and cannot be 



