WORK IN EUROPE 
15 
times. Modern classifications have been modified in consequence 
of newly discovered recent or fossil forms. If we knew in all its 
details the evolution of the successive forms of living beings, classi¬ 
fication would have been replaced by the history of that evolution. 
But as our knowledge of this history is only very fragmentary, the 
practical task of the classifier of the present time consists in contriv¬ 
ing a scheme of arrangement to contain the recent fauna and 
the fragments of the/om7 fauna, so far as known. 
In the “ Life and Letters of T. H. Huxley,” by his son L. Huxley, London, 
1900, Vol. I, p. 312, I find the following passage about fossil remains: “ What a 
wonderful assemblage of beasts there seems to have been in South America ! I sus¬ 
pect if we could find them all they would make the classification of Mammalia 
into a horrid mess.” 
Ever since I began, more than fifty years ago, to reflect upon the 
true aim of classification, I have recognized evolution as a logical pos¬ 
tulate, a sine qua non of organized life. Darwin’s work of 1859 gave 
a new impulse to my thinking, and I gradually came to consider the 
evolution of organic forms through geological ages as the Growth 
of the tree of Life (to use the expression of Genesis iii. 22), whose 
branches and branchlets, under the ambient influences which pre¬ 
vailed under each of the successive epochs, were subjected to the 
regimen of adaptation. Natural selection, according to my view, 
has been the result (or effect ) of adaptation, and not the cause of 
evolution. I have compared the evolution of organic forms to the 
growth of a tree of life. But, as the French say, “ comparaison n’est 
pas raison.” What is Life ? On this question I adhere, at least in 
the present state of our knowledge, to the humble avowal of Pro¬ 
fessor Du Bois-Reymond: “ Ignorabimus ! ” (Du Bois-Reymond, 
“ Ueber die Grenzen des naturwissenschaftlichen Erkennens.” 
Leipzig, 1872, p. 83.) 
Under different conditions of temperature, land, water, and air, 
during geological ages, a multiplicity of terrestrial, aquatic, and 
aerial organizations has been evolved. Among this variety of 
influences, a special mode of life, parasitism , in its various forms, 
became a potent agency in producing aberrant organizations. In 
the class of insects, it is due to parasitism (and especially to the 
internal parasite) that such specialized forms as Rhipiptera and the 
