268 MESSRS. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



To account for these anomalous individuals the following suggestions occurred 

 to us : — 



1. E. esculentus and B. acutus, which are certainly closely related, might not be 

 distinct species at all, but two varieties of one species. 



2. E. esculentus and E. acutus might be two separate species which varied towards 

 one another. 



3. The intermediate forms might be natural hybrids between the two species. 

 Now, considering the ease with which the sperm of the one form will fertilize the 

 eggs of the other in the laboratory, and the fact tliat the two forms are found in the 

 same habitat, in the neighbourhood of Plymouth, and that their breeding periods 

 overlap considerably, there is no reason why hybridization should not occur. Rather 

 it is surprising that hybrids should not be found, as the cross-fertilization is so easy 

 to bring about in the laboratory, and tlie hybrids are quite as healthy as the pure- 

 bred forms. This does not apply, however, to the possibility of natural hybridization 

 between E. miliaris and the other species, since the principal habitat of the former is 

 different. A priori, then, one might expect that hybridization would occur between 

 E. esculentus and E. acutus in the English Channel, near Plymouth. We do not 

 know yet, however, whether hybrids between these species are sterile. It may be 

 that if crossing does occur the hybrid individuals die out after one generation, and 

 that by this means the two species remain distinct. 



With regard to possible natural hybrids there is another point to be considered. 

 The experiments described below show that in general the hybrid larvce produced in 

 the laboratory are not intermediate, but resemble one or the other parent form. If 

 this held for the fully-grown individuals also, it would not be possible to say at sight 

 whether a given individual were a hybrid or not. Tiie metamorphosed hybrids which 

 are at present being reared in the laboratory have shown in some crosses a more 

 marked resemblance to one parent, but in the cross E. esculentus % xE. acutus ^ 

 the resulting hybrid is intermediate between the parent forms. (See Section 15.) 



In a paper recently published {' Echinological Notes') Mortensen describes and 

 figures specimens, one of which is intermediate between E. esculentus and E. acutus 

 var. Flemingii, and the other between E. esculentus and E. miliaris. He suggests 

 that those individuals are natural hybrids between the species mentioned. 



In order to settle, if possible, ^vhether the intermediates found at Plymouth were 

 hybrids between or were vaiieties of E. esculentus and E. acutus var. Flemingii, we 

 sent a number of specimens (photographs of which are reproduced in Plate 25, 

 figs. 108-122) to Dr. Th. Mortensen, who has established the species oi Echinus and 

 their varieties (' Ingolf Echinoidea'). Dr. Mortensen very kindly undertook to make 

 an examination of this material and to give us his opinion on it. The substance of 

 his report is given below. 



In the first place, he does not consider that the statement made in the ' Ingolf 

 Echinoidea' (70), that it is only E. esculentus which carries spines on its buccal 



