456 MFK : OUTLINES OF GENERAL BIOLOGY 



NUTRITION AND REPRODUCTION CONTRASTED.— With 



the preceding saving-clause in view, it is now important to lay 

 emphasis on the contrast between nutrition and reproduction, which 

 is indeed one of the fundamental ideas in biology. Nutrition not only 

 implies fuel for immediate consumption, it implies an increase in 

 the capital of the body, either in the form of stored reserves, or as a 

 fresh growth of tissue. // has cmfyJmtically a plus sign, whereas 

 reproduction is always minus. For reproduction always means 

 parting with some of the living material. The simplest expression 

 of reproduction is seen when a single-celled organism divides into 

 two ; but even in this case the unit must be said to give off half of 

 itself. In an animal like a fish there is an enormous reproductive 

 sacrifice or expenditure, for many hundreds of thousands of egg- 

 cells and still larger numbers of sperm-cells are set adrift into the 

 water. It has been estimated by Mortensen that one of the starfishes, 

 Luidia ciliaris, may liberate two hundred millions of egg-cells in a 

 year. It is not surprising that the reproduction of many types is 

 followed by the death of the individual, as is illustrated from the 

 delicate Mayflies and butterflies to the substantial lampreys 

 and eels. 



Growth and Reproduction. — Growth, as we have seen, is 

 fundamentally a multiplication of the complex molecules that 

 constitute the living cell. In many-celled organisms this also implies 

 the continual multiplication of cells, for apart from occasional 

 giant-cells, growth is always associated with cell-division; so what 

 in one-celled organisms is termed multiplication or reproduction 

 becomes in many-celled organisms their characteristic process of 

 growth. Thus, for organisms that multiply asexually, it is plain that 

 reproduction is discontinuous growth; and one of the reasons- why 

 it must occur is discoverable in cases where a "limit of growth" is 

 reached, that is to .say, a definite size, which is physiologically the 

 optimum for the organism concerned. Beyond this limit of growth 

 there is a setting in of some degree of instability; and this is the 

 prelude to reproduction. The detailed physiology is not yet elabor- 

 ated, but the fact is clear that reproduction t(>nds to occur at the 

 limit of growth, either for the organism as a whole, or for certain 

 parts. In numerous cases, however, the limit of body growth remains 

 indefinite, as in many fishes, reptiles, animal colonies, and plants; 

 and this makes the analysis of reproduction more difficult in the 

 more highly differentiated organisms. Moreover, the periodic 

 occurrence of reproduction cannot be Uo simply interpreted as the 

 direct result of an instability con.sequent on reaching a limit of 

 growth ; for the regulation of the balance of the body becomes very 

 subtle and imjx^rative, involving, for instance, a periodic activation 

 of hormone-producing tissue in the reproductive organs or gonads. 

 Thus while Haeckel's and Herbert Spencer's dictum that "repro- 



