ASPECTS OF BIOLOGY AND METHOD OF BIOLOGICAL STUDY. 37 
few words on palzontological distribution are all that I can nów 
venture on. ; 
The distribution of organized beings in time has lately come be- 
fore us in a new light by the application to it of the hypothesis of 
evolution. According to this hypothesis, the higher groups of or- 
_ ganized beings now existing on the earth’s surface have come 
down to us, with gradually increasing complexity of structure, by 
continuous descent from forms of extreme simplicity which çon- 
stituted the earliest life of our planet. 
In almost every group of the animal kingdom the members 
which compose it admit of being arranged in a continuous series 
passing down from more specialized, or higher, to more general- 
ized or lower forms; and if we have any record of extinct mem- 
bers of the group, the series may be carried on through these. 
Now while the descent hypothesis obliges us to regard the various 
terms of the series as descended from one another, the most gener- 
alized forms will be found among the extinct ones, and the farther 
back in time we go the simpler do the forms become. 
By a comparison of the forms so arranged we obtain, as it were, 
the law of the series, and can thus form a conception of the miss- 
ing terms and continue the series backwards through time, even 
where no record of the lost forms can be found, until from simpler 
to still simpler terms we at last arrive at the conception of a term 
so generalized that we may regard it as the primordial stock, the 
ancestral form from which all the others have been derived by 
descent. * 
This root form is thus not actually observed, but is rather ob- 
tained by a process of deduction, and is therefore hypothetical. 
We shall strengthen, however, its claims to acceptance by the ap- 
plication of another principle. The study of embryology shows 
that the higher animals, in the course of their development, pass 
through transitory phases which have much in common with the 
permanent condition of lower members of the type to which they 
belong, and therefore with its extinct representatives. We are 
thus enabled to lay down the further principle that the individual, 
in the course of its own development from the egg to the fully 
formed state, recapitulates within that short period of time the va- 
rious forms which its ‘ancestry presented in consecutive epochs of 
the world’s history ; so that if we knew all the stages of its indi- 
vidual development, we should have a long line of its descent. 
