3IO 



THE EVOLUTION OF SEX. 



meadow-rue and larkspur respectively, while the species of these 

 very genera show, within narrower limits, similar swings of 

 variation. What we call higher or lower species are thus the 

 leaders or the laggards along one or other of these two hnes of 

 variation. 



Among animals, the importance of the reproductive factor 

 may be illustrated in the most diverse series. Thus the greatest 



step m 



nature, that between the single-celled and 



many-celled animals, bridged as it is by loose colonies some 

 of which are at a very low morphological level, is not due to 



Formation of the Gastmla.— From Haeckel. 



the selection of the more individuated and highly adapted 

 forms, but to the union of relatively unindividuated cells into 

 an aggregate, in which each becomes diminishingly competitive 

 and increasingly subordinated to the social whole. The 

 colonial or multicellular forms, originating pathologically in all 

 probability, may of course have rapidly justified their existence 

 in the struggle for existence, just as unions of many kinds do 

 in human society, but the Protozoa cannot be accused of any 



