Regarding the evolution of the Spanish spoken in Mexico, the Swedish linguist Bertil Malmberg points out that in Central Mexican Spanish—unlike most varieties in the other Spanish-speaking countries—the vowels lose strength, while consonants are fully pronounced. Malmberg attributes this to a Nahuatl substratum, as part of a broader cultural phenomenon that preserves aspects of indigenous culture through place names of Nahuatl origin, statues that commemorate Aztec rulers, etc. The Mexican linguist Juan M. Lope Blanch, however, finds similar weakening of vowels in regions of several other Spanish-speaking countries; he also finds no similarity between the vowel behavior of Nahuatl and that of Central Mexican Spanish; and thirdly, he finds Nahuatl syllable structure no more complex than that of Spanish. Furthermore, Nahuatl is not alone as a possible influence, as there are currently more than 90 native languages spoken in Mexico, and they all contribute to the diversity of accents found throughout the country. For example, the intonation of some varieties of Mexican Spanish is said to be influenced by that of indigenous languages, including some which are tone languages (e.g. Zapotec). The tonal patterns and overlengthening of the vowels in some forms of Mexican Spanish were particularly strong among mestizos who spoke one of the native Mexican languages as their first language and Spanish as a second language, and it continues so today.