If, however, one should take a knife and trace a few shallow geometrical lines and figures on the surface of the blocks, and the object was to make them interfit perfectly on one another, the game would now be more complex. Each block would not now accord with the surface of any other with equal smoothness; and though all might be super-imposed, some would fit well and some ill, as their surfaces coincided or not; and the mating of them, though still simple, would require some skill. If yet again one should carve out the surface of certain blocks into arabesque designs and delicate organic figures, the child's problem would be made almost insoluble. Each surface would require to be matched with a surface corresponding to its own in complex indentations, and it would be hard to find any two which might be brought into contact without breaking off some of their finer points and marring their beauty; and that child might sit long with each ornate block in its hand, and only when by rare good fortune a surface harmonizing with its corrugations was found, could the matching go on satisfactorily. The game so simple when it began with the smooth blocks, would have become an intricate puzzle with the carved. Very crudely, but with some sharpness, would this child's play exemplify the great problem of sexual selection in the different stages of social and individual growth. The primitive savage, running naked in his primeval wood, or the savage who survives in the heart of a civilized society in the form of the pure sensualist and libertine, resembles our flat block. In him, sexual instinct is still so simple and undifferentiated that every female whom, as the naked savage he captures in war, or, as the clothed savage, purchases with his gold, is equally suited to satisfy his needs; for him the search for sex fellowship is neither much more difficult, or more important, than that of the jelly-fish or the hippopotamus, or any creature still in the quadrupedal stage. For persons in this stage of emotional and intellectual development there is no more any complex problem of sexual selection than for the child with its smooth blocks there is a problem of block selection. On the other hand, if we turn to the individuals of our own or past ages who have reached the highest individualized development, emotional as well as intellectual, we shall find them in the matter of sex selection not inaptly to be compared with our most ornate and carved blocks. As the child may hold its flowered block long in its hand unable to mate it without injuring it, so individuals with highly differentiated intellectual and emotional sex needs, not only find the attainment of satisfactory sexual relationships one of the most difficult and important matters of life, but they frequently fail entirely, and are compelled to accept the mutilated life of celibacy, or the more mutilated life of inharmonious union.[58]