xxxvi PREFACE. 



Therefore, with every disposition to acquire information and 

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

 pelled, as in 1849, to confess ignorance of the mode of operation 

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

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

 law, 1 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 plan 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 intelligible idea of homologous parts in such series 

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

 be the manner of operance of the secondary cause in the pro- 

 duction of species, remains in the hypothetical state exemplified 

 by the guess-endeavours of LAMARCK, DARWIN, WALLACE, and 

 others. 



In the lapse of ages, hypothetically invoked for the mutation 

 of specific distinctions, I would remark that Man is not likely to 

 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 

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



1 BADEN POTVELL, quoting from my Work ' On the Nature of Limbs,' 8vo. 1849, p. 

 86, writes: ' To what actual or secondary cause' ('Essays on the Unity of Worlds,' 

 1855, p. 401), instead of, 'To what natural laws or secondary cause the orderly suc- 

 cession and progression of species may have been committed, we are, as yet, ignorant.' 



