Sentence:  Evidence of transitional stages is missing both in the living world and in the "<WSD>Cambrian explosions</WSD>" at Burgess Shale in the Canadian Rockies and at the Chengjiang site in China. It is only proper at this point that we demand hard evidence of transitional stages. To wait millions of years for it to appear is simply a scientism of the gaps. 
Meaning of the word marked with <WSD> tags:  The relatively rapid appearance, during the Cambrian Period around 541 million years ago, of most major animal phyla, as demonstrated in the fossil record.