INTRODUCTION. 3 



authoritative teaching of the professed scientist, 

 inasmuch as it trusts to the development of his 

 more special knowledge. But the public will often 

 with confidence a thousandfold propagate and ex- 

 pand ideas, which the originator with his larger 

 acquaintance with facts has put forth with reserve. 

 " Let truth be fearlessly taught, but let not that 

 called by us a truth to-day be shown ujd as a false- 

 hood to morrow."* Some half truths of the pre- 

 sent day carried to their legitimate consequences 

 have their dangerous sides, and such do efficiently 

 forward the work of the socialist with his " plastidule 

 soul," and his paralysing doctrines of despair. Surely 

 the philosopher who has but few words and teachings 

 to cancel will have the least cause to ask forgiveness 

 of posterity. 



Should science hereafter lift the veil and clearly 

 show us phases through which a passage is effected 

 from form to form, deep will be the interest ; but even 

 such steps when attained will scarcely touch the great 

 question wrapped in the as yet inscrutable secret, of 

 what causes variation at all in organic growths. 



Perhaps the difficulty of explaining what forces are 

 in action, and what determines the change from the 

 lithesome greyhound to the sturdy mastif , or the swift 

 carrier to the heavily cropped pouter-pigeon, is as 

 great as the task of showing the cause of a specific 

 transmutation, should such a passage ever prove more 

 than a theory. Darwin has well shown that even 

 vertebrae are sometimes lost in varieties produced by 

 the artificial breeding of pigeons ; and who shall say 

 that the laws in operation here may not in ages past 

 have produced by a varied intensity, distinct results 

 in specific development ? 



The cause of the differentiation of parts, and the 

 marvellous selective and aggregative power of cells 

 round definite centres as yet is beyond our ken, and 



^ Address delivered in 1877 at the Munich Meeting of the German 

 Association, by Prof. Rudolf Virchow, of Berlin. 



