392 THE POPULAR SCIENCE MONTHLY 



structure produced. What multitudes of animals are made out of 

 essentially the same hinds of tissues ! How limited in number those 

 kinds are ! The plants show the same lack of cellular diversity. Evo- 

 lution has proceeded by means of new arrangements rather than new 

 materials. This cellular stability, well fitting the needs of organisms, 

 must have been fostered by selection. The nerve cell, the striated 

 muscle cell, are astonishingly modified from ameboid ancestors, but 

 the power that could do so much has left us only a few masterpieces. 

 Is this the result of orthogenesis? Did development proceed along 

 these lines, looking neither to the right nor to the left ; or did selection 

 oppose impassable barriers? Perhaps both, since orthogenetic trends 

 may themselves be favored by selection. 



Passing from tissues to organs and characters, we seem to find 

 much greater, almost infinite, diversity. Eecent research has, however, 

 indicated the presence of determiners in the germ-plasm, factors which 

 may be combined in endless ways in inheritance, but are themselves 

 remarkably stable. It seems nearly certain that, so far from constantly 

 presenting heritable variations, they rarely do so. This conclusion is 

 based not merely on the Mendelian phenomena observed by experi- 

 menters, but also on the paleontological evidence. There are many 

 groups of species, such as the oysters and the oaks, which have existed 

 since Mesozoic times, producing innumerable species, but so far as we 

 can see, practically all by the shuffling of characters present within the 

 genus all along. Among the Unionidas, the fresh-water mussels, Ort- 

 mann has recently commented on the occurrence of practically identical 

 shell characters in different genera; while land snails afford a number 

 of similar instances. In insects, these phenomena are constantly ob- 

 served ; types of color and marking are nearly the same in Lepidoptera 

 of diverse structure; and in some of the Hymenoptera peculiar charac- 

 ters, such as spines on the cheeks, appear here and there as if at random. 

 In one genus of bees the sexes differ in the character of the tongue, one 

 having that organ pointed, the other having it obtuse ; differences hith- 

 erto considered to mark families. 



We are, therefore, led to see a certain stability amidst all the insta- 

 bility of the multicellular animals; a stability of types of tissue on the 

 one hand, a stability of determiners on the other. Change in the stuff 

 of which living things are made is not a common phenomenon ; indeed, 

 we know little or nothing about it. The experiments of Tower and 

 MacDougal, in which heritable variations were apparently produced, 

 can be explained rather on the supposition that certain determiners 

 were destroyed than on the idea that they were altered. 



Natural selection, it has often been said, creates nothing. It merely 

 makes use of the variations already present. In Darwin's time, it was 

 not appreciated that so many of the observable variations are due to the 



