xxxvi PREFACE. 



Therefore, with every disjiosition to acquire information and 

 receive instruction as to how sjjecies become such, I am still com- 

 l)elled, as in 1849, to confess ignorance of the mode of oj)eration 

 of the natural law or secondary cause of their succession on the 

 earth. But that it is an ' orderly succession,' or according to 

 law,' and also 'progressive' or in the ascending course, is evident 

 from actual knowledge of extinct species. 



The inductive basis of belief in the operation of natural law or 

 ' secondary cause ' in the succession and progression of organised 

 species, was laid by the demonstration of the unity of jjlan under- 

 lying the diversity of animal structures, as exemplified by the 

 determinations of special and general homology; by the discovery 

 of the law of ' Irrelative repetition ; ' by observation of the ana- 

 logies of transitory embryonal stages in a higher animal to the 

 matured forms of lower animals ; and by the evidence that in the 

 scale of existing nature, as in the development of the individual, 

 and in the succession of species in time, there is exemplified an 

 ascent from the general or lower to the particular or higher con- 

 dition of organism. 



The most intelligiljle idea of homologous parts in such series 

 is that they are due to inlieritance. How inherited, or what may 

 be the manner of operanee of the secondary cause in the iiro- 

 duction of species, remains in the hypothetical state exemplified 

 by the guess-endeavours of Lajiarck, Daum'ix, Wallace, and 

 others. 



In the lapse of ages, hypothetically invoked for the mutation 

 of specific disthrctions, I would remark that JMan is not likelvto 

 preserve his longer than contemporary species theirs. Seeing 

 the greater variety of influences to which he is subject, the 

 present characters of the human kind are likely to be sooner 

 clianged than those of lower existing species. And, with such 



> Pjaiien PowEi.i,, quutijig from my Work ' On tlie Nature of Limbs,' 8to. 1849, \\ 

 8fi, wriirs:— ■ To what acluiil or socomlary cause' ('Essays on tlio Unity uf 'Worlds,' 

 18')5, p. -nil), insti-iid of, ' To wliat natural la-srs or sr-coudury cause the orderly suc- 

 cession and progrc'ssion of species may luire Lcen committed, ive are, as yet, ignorant.' 



