THE DAWN OF THE EVOLUTIONARY PERIOD. 229 



between different species of animals was a probable cause 

 of modification. Again, he appears to ascribe to the 

 desires of the individual a greater power in producing 

 modifications of its structure than modern zoologists 

 would be disposed to allow. On this subject his views 

 are unfortunately not as unequivocal as might be wished, 

 since, though he uses the word ' desires,' it is not clear 

 that he does not really mean the needs of the animal, in 

 which case he would only mean what Lamarck in his 

 writings understood by the term 'besoins.' Erasmus 

 Darwin's views on this point are so interesting and 

 remarkable that we may quote in full the passage in 

 which they are set forth : 



' From their first rudiment, or primordium, to the 

 termination of their lives, all animals,' he remarks, 

 'undergo perpetual transformations; which are in part 

 produced by their own exertions in consequence of their 

 desires and aversions, of their pleasures and their pains, 

 or of imitations, or of associations ; and many of 

 these acquired forms or propensities are transmitted to 

 their posterity. 



'As air and water are supplied to animals in sufficient 

 profusion, the three great objects of desire which have 

 changed the forms of many animals by ( their exertions 

 to gratify them, are those of love, hunger, and security. 

 A great want of one part of the animal world has 

 consisted in the desire of the exclusive possession of 

 the females; and these have acquired weapons to 

 combat each other for this purpose, as the very thick, 

 shieldlike, horny skin on the shoulder of the boar is 

 a defence only against animals of his own species, 



