Members of a Species Work Together

In the animal world we have seen that the vast majority of species live in societies, and that they find in association the best arms for the struggle for life: understood, of course, in its wide Darwinian sense—not as a struggle for the sheer means of existence, but as a struggle against all natural conditions unfavourable to the species. The animal species, in which individual struggle has been reduced to its narrowest limits, and the practice of mutual aid has attained the greatest devel...

 The Journey of a Fossil

One hundred million years ago, an ammonoid lived in the sea that then separated India from Asia. It died and fell into limy sediments on the seafloor. These sediments grew deeper and hardened into rock. The shell calcified, becoming part of the rock, though maintaining every detail of its structure. India was on the move, drifting on a slab of the Earth's mobile crust toward Asia. The floor of the intervening sea was forced under the Asian continent, back into the hot interior of the planet. ...

 Barriers that Keep Different Species from Interbreeding

What keeps members of two related species from mating with each other? There are many different reproductive barriers. Species might not interbreed simply because their mating or flowering seasons don’t overlap. Some corals, for example, reproduce only one night a year, spewing out masses of eggs and sperm into the sea over a several-hour period. Closely related species living in the same area remain distinct because their peak spawning periods are several hours apart, preventing eggs of on...