CONSTRUCTION OF THE PEDIGREE. 3 
or stage of the system (for example a class, or an order) 
comprises a number of larger and stronger branches of the 
pedigree ; every narrower and lower category (for example 
a genus, or a species) only a smaller and thinner group of 
twigs. It is only when we thus view the natural system as 
a pedigree that we perceive its true value. (Gen. Morph. ii. 
Plate XVII. 397.) 
Since we hold fast this genealogical conception of the 
Organic System, to which alone undoubtedly the future of 
classificatory Zoology and Botany belongs, we should now 
turn our attention to one of the most essential, but also one 
of the most difficult, tasks of the “non-miraculous history of 
creation,” namely, to the actual construction of the Organic 
Pedigree. Let us see how far we are already able to point 
out all the different organic forms as the divergent descend- 
ants of a single or of some few common original forms. 
But how can we construct the actual pedigree of the 
animal and vegetable group of forms from our knowledge 
of them, at present so scanty and fragmentary ? The answer 
to this question lies in what we have already remarked of 
the parallelism of the three series of development—in the 
important causal relation which connects the palzeontolo- 
gical development of all organic tribes with the embryological 
development of individuals, and with the systematic de- 
velopment of groups. 
In order to accomplish our task we shall first have to 
direct our attention to paleontology, or the science of petri- 
factions. For if the Theory of Descent is really true, if the 
petrified remains of formerly living animals and plants 
really proceed from the extinct primeval ancestors and 
progenitors of the present organisms, then, without any- 
