No one has ever seen it happen. It cannot be measured, or demonstrated in a laboratory, and it is also outside the realm of science. I am pretty well convinced that all of the hours I have spent in conversations regarding evolution were probably in vain - for my opponents as well for me. I'm sorry, but none of those examples fit the scientific definition of a transitional form. The differences in the average height of human beings, and the minute differences in the size of the brain cavity in human skulls, are nothing more than evidence of something we see in nature all the time, namely the adaptation of any and all species within each separate and distinct genesis kind.
Japanese people are on the average much smaller in stature than people of western Europe.
Chinese people on the average tend to be taller, and larger than Japanese, yet both are Asians - and again, on the average, both are smaller than western Europeans. Nature allows for all sorts of adaptation within each genesis kind, and these adaptations can produce almost limitless variations of that kind. There is a very distinct difference between a Doberman Pincer and a Russian Wolfhound, but both are members of the same kind. They are dogs. If they chance to crossbreed, their offspring will also be dogs.