1899] THE SCOPE OF NATURAL SELECTION 195 



because it is suited to its environment; (2) because it can repro- 

 duce similar organisms in sufficient number to maintain or increase 

 its relative position in its environment. The more perfect the 

 organism the less its chance of elimination, consequently so long as 

 its reproductive power is successfully maintained it is to its advantage 

 if it can reduce to a minimum the loss incurred by the organism in 

 successful reproduction ; it will follow, therefore, that the cells which 

 throw off least reproductive material from the adult structure will 

 require less nutriment, and therefore the collective organism will, 

 other conditions equal, survive under competitive conditions. For this 

 reason protoplasmic growth will be reduced as far as possible when 

 beyond the needs of the organism, and the reproductive buds or units 

 from each cell will tend to be reduced both in size and number. For 

 these reasons it would obviously be of advantage if merely the mor- 

 phological elements were extruded from the different cells, 1 and these 

 when collected in the reproductive centre would form the material for 

 the new individual. 



6. As differentiation of reproductive function continued run- 

 ning a parallel course with other specialisations of structure, natural 

 selection continuing to favour the best -formed individual and off- 

 spring that environments could allow, two tendencies would become 

 manifest : (1) a tendency to reproductive economy, by which every 

 unnecessary development would be eliminated so as to make reproduc- 

 tion a less and less expensive process to the organism ; (2) owing to 

 increased complexity, specialisation, and evolution of structure, repro- 

 duction would become a more and more delicate process, and would 

 constantly have to be conducted with increasing care, and the stages 

 of development of the organism would therefore become increasingly 

 prolonged. The development of the individual, and the capacity of 

 that individual when developed for competition with other individuals, 

 would form two partly competing and partly complementary elements 

 of race progress, and the resultant of the two would correspond 

 to the line of progressive adaptation and development. With the 

 increasing length of the period of development differentiation of sex 

 ■becomes first an advantage and then a necessity. 



7. A progressively specialised method of food supply will be 

 required to keep pace with the other specialisations. 



In applying these conceptions to the interpretation of phenomena, 

 certain points must be specially emphasized : — 



(«) Every important specialisation of structure must be represented. 



(6) As, however, one of the causes of evolution of structure 

 is quantitative complexity, it follows that every quantita- 

 tive element need not be represented, but only the right 



1 In the extrusion of the polar bodies from the ovum, we may possibly have an instance 

 of what on a smaller scale is universal anions; multicellular organisms. 



