1895- PALEONTOLOGY AND BIOGENETIC LAW. 307 



one of the more lowly organised vertebrates. Certainly we still know 

 too little about the skeleton of the Mesozoic mammals, and especially 

 the Allotheria, for us to form a final opinion on this point. But the 

 warmest adherents of the theory of descent must at all events admit 

 that extinct links between the different Classes and Orders of the 

 vegetable and animal kingdoms are forthcoming only in a small and 

 ever-diminishing number. 



Nevertheless, in the larger groups we know numerous series of 

 forms, which not only bear witness to the great plasticity and 

 adaptibility of their members, but also in their chronological order 

 indicate the line along which modification has taken place in course of 

 time. To be sure, much uncertainty and the personal equation of 

 the authors attach to those genealogical trees that are based entirely 

 on the morphological comparison and determination of the chrono- 

 logical sequence of the forms met with. " It is easy to accumulate 

 probabilities, hard to make out some particular case in such a way 

 that it will stand rigorous criticism," was Huxley's caution so long 

 ago as the year 1870, in his classic address to the Geological Society 

 of London ; and one of the most spirited veterans in the field of 

 mammalian palaeontology decides in his last exhaustive monograph on 

 the fauna of Egerkingen, that the creaking and crackling of leaves and 

 branches already decayed does not encourage one to set foot in the 

 hastily-explored forests of phylogenetic trees. None the less does the 

 tracing of the hidden bonds of relationship exercise a fascinating 

 charm over every investigator. All of us, indeed, are convinced that 

 the mutual relationships of the extinct and still-living members of any 

 large group of organisms may be represented, not in the form of an 

 entangled network, but in that of a much-branched tree. 



In addition to the above facts there is still another series of 

 phenomena which confirms the genetic connection of the palaeonto- 

 logical chains of forms, and this was first observed, strangely enough, 

 by one of the most distinguished opponents of the theory of descent. 

 Louis Agassiz certainly regarded the fossil embryonic types as 

 creative attempts which prophetically foreshadowed genera that 

 appeared later with more mature characters. Fossil creatures with 

 persistent youthful and even embryonic characters could not fail also 

 to be noticed by the adherents of the theory of descent, but were 

 regarded as favouring a view which recurred in very different forms 

 in the philosophical literature of the first decades of this century, 

 and which has lately been precisely formulated by our great German 

 zoologist, Ernst Haeckel, under the name of the " Biogenetic Law." 

 According to this, the development or ontogeny of each individual is 

 only a short recapitulation of the long course of ancestral history 

 (phylogeny) of the species or family in question. There must, there- 

 fore, also be chronological series of fossil embryonic types which 

 would correspond with the different stages in the development of a 

 subsequently existing form ; indeed, the separate divisions of a 



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