INTRODUCTION. 5 



ence of the transmission of properties, and recognise in it a 

 phenomenon of general prevalence, which may indeed present 

 modifications of, but never exceptions to, certain definite laws. We 

 may deduce it from the conditions involved in propagation, and thus 

 explain it to a certain degree; for it is clear that portions of an 

 organism, if they give rise to a new organism, will carry on to it 

 the peculiarities which the primitive organism possessed. This is 

 clearest in the lower organisms, which are propagated by mere 

 division. Each portion forms at once an organism like the first.- 

 But from this there extends a continuous series of methods of pro- 

 pagation, up to those in which generative products come into 

 action, which are quantitatively very different, although in all cases 

 derived from the division of the parent organism. 



The new organism in this case also represents in actual sub- 

 stance the continuation of the ancestral, and will therefore possess 

 qualities which agree with those of the latter. 



The amount of similarity or agreement in the organisation of 

 animals is very various. We recognise animals which differ 

 from one another by slight points only ; then those which are 

 separated by considerable differences ; and again others which, in 

 external or internal organisation, present the greatest differences. 

 Thus agreement, as well as variation, is found in interminable 

 gradations. We call things which are more or less like to one 

 anothei', " related ; ' ; and in like manner, when organisms exhibit 

 likeness, we use that word to denote the reciprocal connection, but 

 in this case we give to it its full meaning of blood-relationship. 



We recognise similar organisms as related to one another, 

 when we can explain the similarity of the organisation by common 

 inheritance. But the degree of this similarity measures the degree 

 of relationship which we can deduce from it. Relationship can be 

 regarded as close when the differences are slight ; while when the 

 differences are great it must be regarded as more distant. We thus 

 substitute for the conception of the agreement, or likeness, of the 

 organisation, that of relationship, for we regard the agreements 

 which obtain in the organisation of a collection of organisms as 

 inherited peculiarities. 



The doctrine of the Blood-relationship of Organisms orPhy- 

 logeny is based on the law of inheritance. Comparative anatomy 

 thus reveals the relations of affinity within the various divisions of 

 the Animal Kingdom by pointing out what is alike and what unlike. 

 [A full account of this most important law of inheritance and 

 its pheenomena is to be found in Hackei/s luminous essay on the 

 subject (Grenerelle Morphologie, vol. ii. p. 170).] 



