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The diaphragm is the large flat muscle. hope that helps :)
The former means those most fit, those with the best set of skills and features for the given environment, will live to pass on the said features in the offspring (with ants, for example, being incredibly fit and adaptable creatures). The latter suggests there is an objective way to measure the "meaningfulness" of each species and having only those most meaningful survive. But this differs from species to species - for a house cat, the great white shark is utterly meaningless.
Also, the meaningful animal has no actual advantage over another to help him survive, whereas fittness is exactly that - it is the ability to survive, ability to pass on your genes.
While I understand it may seem so in the case of domestic animals that the most meaningful to the human are those allowed to reproduce, it is actually the same law: those most adapted for human purpose (which, of course, in the given case means the most meaningful) ARE the fittest here in the human-controlled environment.
