XVI] THE PROBLEM OF PHYLOGENY 1023 



in this connection, of a principle of heredity. And though I have 

 tried throughout this book to lay emphasis on the direct action of 

 causes other than heredity, in short to circumscribe the employment 

 of the latter as a working hypothesis in morphology, there can still 

 be no question whatsoever but that heredity is a vastly important 

 as well as a mysterious thing ; it is one of the great factors in biology, 

 however we may attempt to figure to ourselves, or howsoever we 

 may fail even to imagine, its underlying physical explanation. But 

 I maintain that it is no less an exaggeration if we tend to neglect 

 these direct physical and mechanical modes of causation altogether, 

 and to see in the characters of a bone merely the results of variation 

 and of heredity, and to trust, in consequence, to those characters 

 as a sure and certain and unquestioned guide to affinity and phylo- 

 geny. Comparative anatomy has its physiological side, which filled 

 men's minds in John Hunter's day, and in Owen's day; it has its 

 classificatory and phylogenetic aspect, which all but filled men's 

 minds in the early days of Darwinism; and we can lose sight of 

 neither aspect without risk of error and misconception. 



It is certain that the question of phylogeny, always difficult, 

 becomes especially so in cases where a great change of physical or 

 mechanical conditions has come about, and where accordingly the 

 former physical and physiological constraints are altered or removed. 

 The great depths of the sea differ from other habitations of the 

 living, not least in their eternal quietude. The fishes which dwell 

 therein are quaint and strange; their huge heads, prodigious jaws, 

 and long tails and tentacles are, as it were, gross exaggerations of 

 the common and conventional forms. We look in vain for any 

 purposeful cause or physiological explanation of these enormities; 

 and are left under a vague impression that life has been going on 

 in the security of all but perfect equilibrium, and that the resulting 

 forms, liberated from many ordinary constraints, have grown with 

 unusual freedom*. 



To discuss these questions at length would be to enter on a 

 discussion of Lamarck's philosophy of biology, and of many other 

 things besides. But let us take one single illustration. The affinities 

 of the whales constitute, as will be readily admitted, a very hard 

 problem in phylogenetic classification. We know now that the 



* Cf. supra, p. 423. 



