INTRODUCTION. xlvii 



Subsequently to the researches above-mentioned, the late 

 Capt. Du Cane investigated the development of the shore 

 crab, Carcinus Manas, and of the Ditch Prawn, Palcmon 

 varians, with complete success ; establishing in each of 

 these forms the truth of Mr. Thompson's position. Mr. 

 H. Goodsir also examined, with similar results, the former 

 species. But by far the most complete illustration of the 

 subject and the most extensive proofs of the general 

 law, are afforded by the researches of my friend, Mr. 

 Richard Q. Couch, of Penzance, who, dissatisfied with 

 the uncertainty and contradiction of former testimony, 

 resolved to investigate the matter for himself; and this 

 he effected with a degree of acumen and perseverance 

 which characterise all his researches, and by which the 

 truth of the doctrine is fully established, as regards 

 the genera Cancer, Zantho, Pilumnus, Carcinus, Por- 

 titnm, Polybiits, Maia, Gahithea, Homarns, and Pali- 

 nurus a goodly number to have been investigated by 

 one observer and of some of these he watched every 

 change. These results were published in two Memoirs, 

 read to the Cornwall Polytechnic Society in 18-13; in 

 which the author takes a clear and fair view of the whole 

 subject, and comes to his decision with a host of evidence 

 sufficient to set the substantive question entirely at rest. 

 Unfortunately, the useful local publication in which these 

 memoirs appeared, is so much confined in its circulation 

 that it has probably fallen into the hands of but few 

 naturalists. 



I have felt it desirable to give a more extended history 

 of the discovery, as, with the exception of Mr. R. Couch's 

 first memoir just referred to, no such digest has ever been 

 placed at one view before the world. I now proceed to 



