i8 9 6. NOTES AND COMMENTS. 149 



In the first place, it is clear that, whether it be a question of mere 

 mass of life or of variety of life, haul for haul, the shallow waters hold 

 their own against the deep sea, and form, in all probability, the most 

 prolific zone of life on this globe. From the lists which are given it 

 is seen that the species included in each haul belong in each case to a 

 relatively large number of genera. Thus, in one case, the 112 species 

 found were referred to 103 genera, while in a smaller haul thirty-nine 

 species belonged to thirty-four genera, and so on. " These figures," 

 says Professor Herdman, " are particularly interesting in their 

 bearing on the Darwinian principle, that an animal's most potent 

 enemies are its own close allies. Is it then the case, as the above- 

 cited instances suggest, that the species of a genus rarely live 

 together ; that if in a haul you get half-a-dozen species of lamelli- 

 branchs, amphipods, or annelids, they will probably belong to as 

 many genera, and if these genera contain other British species these 

 will probably occur in some other locality, perhaps on a different 

 bottom, or at a greater depth ? It is obviously necessary to count 

 the total number of genera and species of the groups in the local 

 fauna, as known, and compare these with the numbers obtained in 

 particular hauls. That has been done to some extent with the 

 ' Fauna ' of Liverpool Bay." The conclusion thus obtained is that a 

 disproportionately large number is represented by the assemblage of 

 species at any one spot, which means that closely-allied species are, 

 as a rule, not found in association : birds of a feather do not flock 

 together. Is it not, however, possible that this fact is a necessary 

 consequence of association or absence of isolation ? Allied species of 

 Invertebrata would, if in association, tend to merge into one species 

 through inter-crossing. There are, of course, a few cases of allied 

 species occurring together. Sometimes this may be due to some special 

 habit which, although the species are allied forms, prevents them from 

 being severe competitors. Even sessile animals, such as hydroids and 

 bryozoans, appear to be subject to the same rule, although Professor 

 Herdman suggests that this may act to a less extent in their cases. 



In connection with this subject, it would be very interesting if 

 palaeontologists would enquire whether the assemblages of fossils 

 found in various deposits, such, for example, as the well-known slabs 

 of the Wenlock Limestone, showed a similar admixture of genera. 

 It is certainly a curious fact that the beautiful slabs from the Wenlock 

 Limestone of Dudley, containing fossil crinoids, which are now 

 exhibited in the Geological Department at the British Museum, 

 contain almost as many genera as species. One slab, for instance, 

 little larger than a man's hand, contains portions of eight crinoid indi- 

 viduals, belonging to eight species and seven genera. Another small 

 slab from the Kinderhook beds of Iowa contains nine individuals, 

 distributed among six species and four genera. This is very 

 different to the crinoid slabs of Secondary age exhibited in the 

 museum galleries, for in their case there is rarely more than one 



