CH. IV] LIFE IN THE SEA 91 



organisms are incredibly fertile. In a single tapeworm inhabiting 

 the intestine of the skate or cod there may be hundreds of separate 

 segments or joints and each of these is a sexual unit and may 

 produce many hundreds or even thousands of eggs. When we 

 consider the unicellular organisms we are no less impressed with 

 their fertility. A very small mass of diatoms put into a dish with 

 a little sea-water and a piece of weed and exposed to the sunlight 

 for a day or two multiplies to an astonishing extent. All uni- 

 cellular organisms W'hich reproduce by simple division do so at 

 times with great rapidity. Given the proper conditions of 

 nutrition and a few infusoria contained in a little mud will in a 

 very short time have produced an incredible progeny. One single 

 marine bacterium will even at the ordinary temperature multiply 

 to the extent of several millions in the course of a week or two. 



On the other hand the life-histories of some marine animals 

 are complete antitheses to those indicated above. Whales, seals, 

 porpoises and other marine mammalia are like their terrestrial 

 allies slowly-breeding creatures. The viviparous dog-fishes produce 

 few offspring in the course of the year. Those fishes, like the 

 skates and rays, which lay large yolked eggs only deposit a few 

 dozen at the most in the breeding season. Pelagic worms and 

 Crustacea are not nearly so prolific as the demersal forms. The 

 viviparous invertebrata produce few embryos in the year. 



Now it is not difficult to explain why it is that such differences 

 in the fertility of marine animals should exist. It is an apparent 

 paradox that while a turbot produces annually nine millions of eggs, 

 and a ray only a dozen or two, the ray should be much more 

 abundant and more widely distributed than the turbot. But 

 there is no necessary relation between the fertility of a marine 

 organism and its abundance. " No fallacy," says Darwin, '' is 

 more common among naturalists, than that the numbers of an 

 individual species depend on its powers of propagation^." All 

 that the fertility of such fishes as the turbot and cod, or such 

 molluscs as the mussel, or Crustacea like the shore-crab, indicates is 

 that the destruction of these creatures at some stage of their life- 

 history is enormous — just as enormous as their fertility is great. 

 And conversely, the fact that a fish like the ray or dogfish, or a 



1 Voyage of the Beagle, Chap, ix. 



