14 THE PHYLOGENETIC SERIES : [iktrod. 



when even these splendid observations cannot be made to shew 

 much more. Surely their use is now rather to point the direction 

 in which we must go for more facts. 



The questions which by the Study of Variation we hope to 

 answer may be thus expressed. In affirming our belief in the 

 doctrine of the Community of Descent of living things, we declare 

 that we believe all living things to stand to each other in definite 

 genetic relationships. If then all the individuals which have 

 lived on the earth could be simultaneously before us, we believe 

 that it would be possible to arrange them all, so that each stood 

 in its own ordinal position in series. We believe that all the 

 secondary series together make up one primary series from which 

 each severally arises. This is the fundamental conception of 

 Evolution and is represented figuratively by the familiar image of 

 a genealogical tree. If then all the individual ancestors of any 

 given form were before us and were arranged in their order, we 

 believe they would constitute a series. This view of the forms of 

 organisms as constituting a series or progression is the central idea 

 of modern biology, and must be borne continually in mind in the 

 attempt to apply any principle to the Study of Evolution. 



Each individual and each type which exists at the present 

 moment stands, for the moment, therefore, as the last term of 

 such a series. The problem is to find the other terms. In the 

 case of each type the question is thus stated in a particular 

 form, and it is a somewhat remarkable circumstance that it is in 

 its particular forms that this problem has been most studied. 

 The same problem is nevertheless capable of being stated in the 

 general form also. Instead of considering what has been the 

 actual series from which a specified type has been derived, we may 

 consider what are the characters and attributes of such series in 

 general. It may indeed be contended that it is scarcely reason- 

 able to expect to discover the line of descent of a given form, for 

 the evidence is gone ; but we may hope to find the general 

 chararacteristics of Evolution, for Evolution, as we believe, is still 

 in progress. It is really a strange thing that so much enterprise 

 and research should have been given to the task of reconstructing 

 particular pedigrees — a work in which at best the facts must be 

 eked out largely with speculation — while no one has ever seriously 

 tried to determine the general characters of such a series. Yet if 

 our modern conception of Descent is a right one, it is a pheno- 

 menon now at this time occurring, which by common observations, 

 without the use of any imagination whatever, we may now see. 

 The chief object, then, with which we shall begin the Study of 

 Variation will be the determination of the nature of the Series by 

 which forms are evolved. 



The first questions that we shall seek to answer refer to the 

 manner in which differentiation is introduced in these Series. 



