Almost half the birds in the yard were brown cardinals and the rest were bright red cardinals, so jimmy perceived them as two distinct groups of birds. this best illustrates the principle of:

The distinction of two groups of birds (brown cardinals and bright red cardinals_ best illustrates the principle of similarity. According the principle of similarity things which share visual characteristics (size, shape, color, texture, ...) are belonging together .

It is used to during sexual reproduction between humans