PaUvontology and the Biogenetic Law. — von Zittel. 143 
and orders of the vegetable and animal kingdoms are forth- 
coming 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 chrono- 
logical 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 morphologi- 
cal comparison and determination of the chronological se- 
quence 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 ci'iticism," 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 spirit- 
ed 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 ph3dogenetic trees. None the less does the 
tracing of the hidden bonds of relationship exercise a fasci- 
nating 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 pahie- 
ontological 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 pro- 
phetically foreshadowed genera that appeared later with more 
mature characters. Fossil creatures with persistent j'^outhful 
and even embryonic characters could not fail also to be no- 
ticed by the adherents of the theory of descent, but were 
regarded as favoring 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 
