Increases of some structures appear to parallel decreases in others
It seems odd, but no animal with horns or antlers, whether fossil or recent, has all the teeth required to give it the complete and full dentition characteristics of mammals. No species of spider is known to possess the ability to sting, but all have a poisonous bite (although, luckily only a few possess poisons potent enough to harm humans). The most colourful birds like tropical parrots and Papua New Guinea birds-of-paradise leave a lot to be desired when it comes to their ability to sing and the increase in swim speeds of fish like the tuna apparently went hand in hand with the loss of the buoyancy organ, the swim bladder. There are lots more of such examples, but what am I trying to demonstrate with these examples? I’m trying to show that the development in one area frequently has consequences in another and that a “push” in one evolutionary direction can see a reversal in another. —>—>
Have you ever heard of “pseudocyesis”?
Teachers can have an enormous impact on whether a child likes a subject or not. I, for instance, hated math at middle school, but with a new teacher at high school I began to love math to an extent that I wanted to study it at university. History was another subject, whose teacher killed my early interest in it, because he demanded that we kids byhearted hundreds of historic dates. Had he told us of the pseudo-pregnancies (also known as false or phantom pregnancy and pseudocyesis) of Queen Mary in 1555, Queen Draga of Serbia in 1900 and the Zar’s wife Alexandra in 1903, history classes could have been so much more interesting! —>
They’re not exactly pleasant behaviours
Has it ever happened to you that when you wanted to fry an egg and you cracked it open you found two egg yolks in it? If you had that experience you would probably have wondered whether two chicks could have developed from such an egg. Fact is that although two embryos may initially grow, one soon overtakes the other and causes its death, so that in the end only a single chick actually hatches. Can embryos really be so murderous?